Craigslist.org is an awesome site for spare parts and machines. Also check schools, they sometimes give away old computers. I'm working at a tech department at a school and we usually give away about 15 old computers and a bunch (ok more like a truck full) of spare parts.

Oh, and first mod... Hmm... I think it was adding a USB Hub to an old keyboard. Was pretty simple, just drill a few holes here and there and tape some wires toghether.