You have to run them in series to get 36 volts. Google it for a drawing. BUT, adding 12 volts usually doesn't affect the live of the motor...it's the controller that you might fry. That said I run my 24V hub on a 36V NiCad and it works fine, but it's not an iZip.
Most battery sales shops have small 12V SLA batteries for about $35. You may have to find a charger.
One last thing: If you are uinhappy with the performance of the bike you might be better off flogging it on craigslist and getting a hub motor kit. Something like a 36V Crystalyte will give you neck bending acceleration and a higher top speed into the wind and up hills.