You'd need special hardware to test each cell. Something like a super brain 989:
super brain 989 | eBay
I expect you could get cells on eBay, but that's pretty sketchy and they would probably not survive the first time you pressed on the brakes. A better way would be to contact Eli over at InsightCentral - he's the guy behind BumbleBee Batteries. I'm sure he'd be happy to sell you a few good sticks.
Better to try grid charging first though, and see what you can get out of the pack. If you don't feel comfortable building a charger, get the one Balto linked. You'll want a charger for battery maintenance, even if you end up replacing the battery.