[link|http://www.hardwaresecrets.com/printpage/81/1|Hardware Secrets] has info on the various battery types.
The "coin" type batteries can sometimes be found at your favorite drugstore (e.g. in the hearing aid battery section). Otherwise, scrounge around [link|http://www.google.com/products?q=motherboard+battery&hl=en&safe=off&rlz=1B3GGGL_enUS219US219&um=1&sa=X&oi=froogle&ct=title|here] once you know what you need.