You can try the Trillian Website Skin Section. Best skins I know of. There's also DeviantART's Trillian and Trillian Pro skins.
As for AIM. AIM doesn't have skins to my knowledge. What AIM does has is *expressions*. You can check out AIM Expressions.com.
Edit by quietbritishjim
The deviantart links have moved, they're now:
Trillian 3 skins and emoticon packs on deviantArt
The official Trillian website hasn't been updated for ages, so that is really the best place to go.
The old skins are still on deviantArt too, they should normally work (despite the warning in Trillian) but they don't include voice/video chat stuff so you won't be able to use those features: