NV, walk into a few of small shops that cell invertors/batteries and ask about a price for used one. There are hundreds, if not thousands of such shops in SD, I seen at least three in Megacentro are, but they are everywhere.
Once you found the battery you are willing to pay for, request to have it checked by tester and under the load in front of you. Tester must read "full" after 20 Sec. of loading the battery (load resistor gets quite hot and you should feel hot air rising from it). If the shop does not have battery tester - do not buy there, no matter how well is the price and how long the warranty, go next door. A couple of hours of shopping should find you what you are looking for.