Amps are a measure of current. Batteries are rated in volts and amp hours (Ah) or milliamp hours (mAh). The amount of time the battery will last is roughly the capacity divided by the amount of current. For example, a 1 Ah battery can supply an amp for about an hour.

The batteries are 12 volts and typically about 5-15 amp hours, which is plenty. A smaller battery will need to be charged more often. A bigger battery will be too heavy to carry around.

Tower Hobbies, Hobbico, etc. sell the batteries but you can probably find them somewhere else a lot cheaper.