Hi!
Motorcycle batteries are easily obtainable in every bike shop. You only have to mention the voltage and Amp, and the hight, lenght and with.
See to that if you buy a lead /acid battery that you buy a maintenance free battery. Not a battery you have to fill with acid...which might tend to leak acid out the overflow tubing.
If you want a ligher (around 1 kg) ,buy a Lipofe4 batteri.