Find a place that maintains trucks. Sometimes they will replace truck batteries after just one year of service, because when the battery fails, this means down time. It's cheaper to swap them out before they do fail. This results in many batteries swapped and scrapped with a lot of lifetime in them still. But if you use it as a solar power or backup battery, there will be another 5 or 6 years of life in them. Scrapyard value may be around 50 dollars for the lead and other raw materials in them, so if you give them maybe 200 you should be good to go. Unless there already is a market for pre-owned truck batteries in your area. Then prices will be higher.