I recently bought a battery for exactly the same reason. It is 12 volt 7ah fully rechargeable.
Unfortunately, I am in Australia so of no real help to you. But, I can tell it works a treat and cost $27 Australian, under $19US!
Brodaks is selling similar for $35. The one I have looks exactly the same and has exactly the same dimensions.
I originally charged it after every flying day but it only went on the charger for 2 minutes! So I left it for 3 flying days and it took 3 minutes. I have a 10 amp 7 stage charger, originally bought for a boat battery, I am not up with electrical stuff so if it is the wrong charger or I am doing something wrong I guess I will find out down the line but at this stage I am very happy with the battery.
Check security companies, these sized batteries are commonly used as household backup security system supply. Also Craigs list? I found mine on Gumtree, a selling platform similar to Craig's list in Australia.
Good hunting
Craig