Also pretty hard to recomend without knowing what you are running. Edit, Duhh, you're the one with a heinzmann.
But, assuming you want to just get more sla's, B&B 's are highly regarded, but the type designed for EV use are so expensive, that the TS lipo, or Hobby King lipo might be almost as cheap. If you only need very short range, you could get by with as small as 5 ah of lipo.
Another popular option for those on tighter budgets, is the Vpower HK lifepo4 on Ebay. Make sure you get a really good price on an auction though, and don't get bid up. A Ping 24v 15 ah would be about $350 including shipping so don't pay more than 300 for the same thing in vpower auctions. Don't try a 10 ah with those 2c lifepo4 batteries, you have a 15 amp controller, so you need at least a 15 amp battery to keep it's discharge rate to where the battery lasts.
If you went lipo, 5000 mha 20c packs, ( 5ah) one 3s and one 4s would get you the perfect voltage series connected. About $65 per 5 ah plus shipping. Then you'd need a charger, power supply and some low voltage warning beepers. Another $100. and shipping So 5 ah would cost you at least $200-225. But then after that, you could start adding 5 ah more for less than $100 each time. As you made the pack bigger, you'd get into more of a hassle to charge too. But the lipo option does get cheaper to expand range once you are set up to charge em. Not that I'm advising lipo, just showing what it could cost to get started if you chose that route. I still think for daily drivers, the ease of charging lifepo4 make it a better choice.
Lastly, one of the cheaper, easier options would be to go to wallmart and buy some batteries for toy electric cars. Not the best, but suitable and about $65 each, and you can have them today. They are comparable to what comes in cheap kits.
