Honestly, any hobbyshop in your area should have batteries. What you want is a 6-cell 7.2V NIMH battery pack. You can bring one of your old batteries with you to the shop, to make sure it will fit. Choose the capacity of your choice; the larger, the better. 3300mAH or larger would be perfect.

The radio may currently be "twitchy" because of the weak batteries. If you try a new battery, and it still "twitches", then you can buy a new radio. I have a feeling a new battery should solve this problem.