look for ebike batteries.
http://www.batteryspace.com/e-bikeskitandbatteries.aspx
the good stuff is Lithium Iron Phosphate, but you're talking $300 for 12v/20ah. But you're around $300 even for NiMH. The only way to get 12v@20ah for cheap is lead acid, where you can pick that up for $50.
The most I've seen i a bottle battery is about 5AH, so you'd need 4 of them to hit 20AH. LiFePO4 bottle batteries hit nearly 7AH, but then you're up to > $100 a pack.