I've seen a few on ebay. I think The Doc has been using pieces from 40K and WHFB (Warhammer Fantasy Battle). You can also look on the main website for Games Workshop, they sell the whole kits and used to sell the bits individually for kitbashing. You would probably be better if you can find a shop that sells "bitz" locally. The kits can be pretty expensive, usually $40 or so unless you get a used one.