How do I replace an old Roomba's battery?
Ebay - that's where I got my replacement. It's NOT an iRobot manufactured battery pack (nor does it claim to be), but it works fine. I think it came from someone in Hong Kong, and the shipping wasn't bad at all - I don't know how shipping in your area would be.

I looked at opening the pack up and replacing the cells, but I couldn't figure out how to get the thing open without damaging it.