Many (but NOT all) the graphics in AI war come from this old DOS game called Tyrian, the author of which has released most of the sprites in it for public use, though I do not know the exact liscencrle.
That may be a good starting pont.
EDIT: Also, if you have never played Tyrian, do so. It is a wonderful vertical shooter that still holds up. The "basic" versions are freeware now. Also, the source code for that version was released and there is a very accurate open source port to C as well.