The manga of X is definitely a better source for Sorashi scenes. There's a lot more gradual development than the anime has, and there's also some bonus mush scenes ~_^. This section is going to have a brief run-down of the Sorashi relationship in the manga. The main manga volumes that you'll want for Sorashi-manga are:

© Volume Five
© Volume Twelve
© Volume Fourteen
© Volume Sixteen
© Volume Seventeen
© Volume Eighteen
© Current ASUKA's

There are also some cute little scenes in other volumes of the manga, you can see these in the scans section of this site. For anyone who wants to know, they met in volume three.

Volume five is basically the beginning stages of their relationship. Sorata flirts with Arashi (and Arashi doesn't appriciate this) throughout the volume, and later protects her when there is a earthquake-of-sorts in the diet building. However, Sorata also takes advantage of the moment ^^;;.
Later on, Sorata and Arashi are watching Kamui from the trees. Arashi asks Sorata about what he meant when he said that he had 'chosen' her. Sorata then tells Arashi about his destiny and that he has chosen her to be the woman he will die for. There's a dramatic moment as Arashi soaks this in, and then Sorata goes all silly and starts hitting on her again. ^^;; Karen then shows up in her gear and the whole moment is over.

Volume twelve has one of the more memorable scenes in the series. You can also see this scene in episode twenty of the anime. Kamui is moping around because of, who else, Fuuma and hasn't really been eating. Sorata goes in and cheers him up, and so Kamui comes downstairs and eats with the rest of their little "family". This is something that strikes a chord with Arashi, who was in a similar state to that of Kamui during her childhood.
Arashi and Sorata are both in the kitchen getting tea, and Sorata stops Arashi and (very seriously) asks if she would have eaten if he had asked her to. He then leaves, and Arashi is left staring at the hand Sorata touched. This is probably one of the earliest reactions that Arashi has to Sorata... *grins*

Volume fourteen of the manga sees the destruction of one of the Kekkai, as Arashi and Sorata watch from CLAMP Campus. The two are talking about this, but the topic soon turns to Kamui. Sorata mentions how easy it is to read Kamui and Arashi looks surprised that Sorata can do this. She then leans down and asks Sorata if he can read her too, to tell her what she's thinking. Sorata looks surprised but then remarks that she's thinking that he's not that serious, but he can really be perceptive.
Arashi gets a surprised look on her face, and Sorata continues that the one thing he can't tell is how she feels about him. He kisses some strands of her hair, and Arashi begins to reply when Nokoru and company interupt them. ^^;; When Sorata turns around Arashi is on the opposite side of the room with her back to him. He goes back to talking with them, and we get a view of Arashi as she turns to watch him and then holds the strands of hair to herself.

By volume sixteen, Arashi has some very obvious feelings for Sorata (who, being a guy, has no clue) and Sorata is still hitting on her. Of course, now it has something of a different affect on Arashi. The two, along with Yuzuriha and Kamui, are with the Nokoru and company, watching the wreckage from the earthquakes in Tokyo.
Sorata makes a joke about how the earth can't be destroyed yet because he and Arashi haven't "done anything" yet, and then he quickly goes to protect himself. Arashi, however, doesn't move or make a comment. Sorata tries a little more light joking, and when Arashi still doesn't say anything, he reaches over to check if her forehead is warm. As soon as his fingers touch, Arashi looks surprised and pushes his hand away, saying that shes fine. Sorata looks a little uncomfortable, and when we see Arashi from the front she's all flushed.
The group decides to break into two (boys and girls) to keep an eye on the different Kekkai, and as Sorata and Kamui leave Yuzuriha looks over to see Arashi sort of gazing toward the door that they left through. Naturally, Arashi gets some questions on this when she and Yuzuriha are at Inokashira. She gets pretty flustered over it too ^^;;;;. At the end of the volume, the two girls feel the presence of a Chi no Ryu.

Kamui and Sorata have begun to catch onto the fact that something is up with Hinoto; meanwhile, Arashi and Yuzuriha are fighting the BEAST. Somehow, Arashi ends up seated on the ground... with the bridge hanging over her head. Arashi is pretty much frozen in place as Yuzuriha shouts a warning. The bridge drops.
As the dust clears, a figure is seen holding up the bridge while Arashi looks on in shock. Hearing Yuzuriha shout, Arashi quickly runs out of harm's way, glancing back at the last second and catching the eye of the creature. Once again the bridge collapses, and the creature is gone. The two work out that it must have been a magical construct, and as it occurs to them that it may have been a certain guy's, Kamui shows up with a very beat up Sorata.
Arashi runs over, crying out and trying to stop the bleeding; Sorata trys to comfort her, but he coughs up blood and then passes out. While Kamui and Yuzuriha pick up Sorata, trying to take him to get help, Arashi stands frozen, remembering Sorata's earlier words about his destiny. She then dashes forward, losing control and grabbing onto Sorata, crying as she trys to wake him up. Kamui manages to calm her down, and there's a scene change to a hospital room on CLAMP campus, where Sorata is all bandaged up and sleeping with Arashi watching over.
Sorata wakes up, takes stock of his injuries, and Arashi (who is crying) asks him if the magical construct was his. Sorata replies the affirmative, and then surprises her by saying that he assigned it to her the day after they met. Arashi breaks down and yells at Sorata for never considering her feelings about these things. Sorata talks about how it is okay that she doesn't have any feelings for him, and that it was his choice, and Arashi tells him that she does have feelings for him.
Sorata looks shocked at this, and brushes back her hair, trying to comfort her. He hugs her and tells her to stop crying and that he's very happy about this, and Arashi replies that she can't control her tears (;_____;). Sorata trys to cheer her up by making a comment about how he got her first time, and Arashi flushes up. Sorata then asks if he can have another first time, and the two kiss. (*squeals* As a fangirl, I must comment that this is adorable.)

There isn't a lot in this volume, but what is here is high-quality stuff. ^_~ Near the beginning of the manga, we get an ominous mention from Kakyou about how there's going to be a new Chi no Ryu, and the image that appears, although hooded, has long, straight dark hair (just like a certain someone's)...
The scene changes back to the hospital room where Sorata and Arashi are. We get a view of birds and feather outside the window, a pile of clothes by the bed, and then the picture. *grins* So we now know what first time Sorata was talking about. *kyehehe*
Arashi wakes up and sits up to look down at Sorata. He's still asleep and looking all innocent and stuff. Arashi smiles and then leans down to give him a kiss on the forehead. She then sits back up and stares at her palm....
A little later on, Kamui and Yuzuriha are coming to visit Sorata. They reach his room and knock on the door, and there's the sound of pounding footsteps and then the door is flung open by Sorata, who's shouting 'Arashi?' He was probably expecting it to be her, poor guy. Kamui wonders if something happened between Sorata and Arashi, and Sorata says that he has a bad feeling.
Scene change to Arashi, who is on top of a building and thinking about the fact that she can no longer produce the sword, just like her mother. Arashi then remembers Sorata's words about his destiny and shivers, then leaves.

In the last X chapter (which was around seven months ago ~_~) Sorata shows up at the Diet Building and demands that Hinoto return Arashi to him. We then see how Arashi had come to see Hinoto about the loss of her abilities, and Hinoto sealed her in the dreamscape. Hinoto calls in her henchmen, and Sorata tells them to step aside because he's going to fight.