Oh and a word of advice, Mac's usually refer to motherboards as Logic boards, if you start looking. You can find some parts and stuff on eb@y, but it seems to me that buying it whole you be cheaper a lot of times than the parts, and also remember that things like power connectors and even slot layout are not standardized, so often there is little you can replace cross-generations, or at least without a decent amount of tweaking/adaptors.
If you do find anything though, let me know...I would love to get a hold of a g3 era mac to play with, but the prices are a little more than my budget right now.
Orie