Are you looking for these in the USA, or another country?
There is nothing special about the 12V sealed lead-acid / absorbent glass mat batteries that were sold with your kit. You just need to find a dealer with a 12V, 12Ah SLA/AGM battery you can buy. There are a lot of possibilities. I have had my eye on Rage Battery (
www.ragebattery.com) but I haven't bought from them, so I can't comment on their reliability, customer service, or warranty support. They are currently selling a case of four (4) 12V, 12Ah batteries for $69.88 shipped
[edit... prices and shipping policies may have changed since this post, post up if you are charged a shipping fee], with a 2-year warranty against manufacturer's defects.
http://www.ragebattery.com/power-sonic/D5775case.htmlOne thing to remember about warranties on batteries. If they are limited cycle batteries, such as NiMH or SLA, you shouldn't expect to be able to replace your batteries under warranty service after you have run them through more than 100 or 150 cycles. Also, deep discharges should be considered warranty voiding. Batteries are a consumable and have a limited cycle life. The main point of the warranty is to protect you against DOA and extremely short lived batteries. Thinking you are going to ride a set of SLA batteries hard for 2 years of free replacements is wishful thinking and is out of the realm of reasonable expectations.
-Mike