Normally I wouldn't suggest a Chinese supplier, but just for interest I purchased a 36V, 10ah battery from Unit Pack Power (Micah Toll, electrek has recommended them at times), for $184 with charger (ebay). They will configure the battery the way you want and shipped it within a month (might have been a lot less, but I didn't need it so don't know for sure). I keep it in an ammo box since I don't trust Chinese batteries fully, but it seems well made to me. Also, as Doc said, depending on your controller, you might be able to use a 36V battery for extra speed and/or distance.