The CMOS battery should just be the battery on the motherboard itself yeah? Should be pretty cheap and easy to do - I had to do it once on one of my old comps. That comp was draining the battery like water in a funnel when the power was off because it would use the battery to power the light in the optical mouse.

Anyway if it's anything like that it's likely just going to be an easy matter of pulling the battery out of it's holder and replacing it with the same type. The one on mine was just something you could buy at walmart or any electroincs store - a battery that was coin shaped although a bit thicker. May want to check and pull out the battery first and take it with you to the store so you can get the right size. Lots of coin-shaped batteries, if that's what it turns out to be.