Well, wherever you are, there are two likely places in your town to find a 12 ah sla suitable for ev use. One is a store that sells a kids toy electric car. Replacement batteries for them are EV grade batteries. The other is a store selling stuff for electric wheelchairs.
Other batteries might be avaliable, but most likely they are intended for other uses, that discharge at a slower rate. They'll be cheaper, but not as good for an ebike use.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.