A set of 12 ah sla's would do ya, but you'd have to ride at less than full speed to keep the discharge depth low enough for them to last. I like to think of 12 ah sla's as good for 5-6 miles at full speed, flat ground. I don't recall your controller, but an aotema (WE) can definitely go 48v in sla or nicad. Make sure the sla's are for running a motor of some kind, not lights or computers. Wall mart toy car batteries are the ideal kind, $50 each.
A good option though, for your distance is nicads. They don't have the limitations of depth of discharge or needing to charge immediate that sla's have. Ebikes .ca stil sells em because for many riders, they are still perfect. They are known to be noob proof. Some of the more recent offerings of headway are still a better deal, though they might cost 50-100 bucks more.
But you won't regret buying a larger battery size since I garantee you, once you have 20 + mile range you will find wonderfull ways to use it. Getting out the plastic, and going for a 36v 15 ah, or 36v 20 ah pack will not be regreted by 99% of buyers. Ping is a great supplier, though pricy. But isn't High Tech Bikes in the bay area? He's a great supplier too, and sells a similar pack to pings. Going bigger with a pouch cell pack gives you longer lifespan than a smaller pouch cell pack. The headways should last in a smaller size, but knowing for sure will take some riding from those who bought em. I have 4500 miles on my 36v 20 ah ping now, and it's looking like 1000 cycles is not going to be hype. We'll know in about a year and a half whether I get more than 10,000 miles out of this pack and still have 15 mile range. ( 23 mile range now full speed)
