In general it's an ad-hoc market, but even eBay is fine for this. Lots of older Nissan and Volt cells available for sale. Challenge being shipping. I got a sing

In general it's an ad-hoc market, but even eBay is fine for this. Lots of older Nissan and Volt cells available for sale. Challenge being shipping. I got a single Gen 1 Volt pack sitting in my shop, waiting for a project, and I got it for a good price off eBay two years ago, from a Canadian supplier so shipping was also domestic and reasonable. Quite handy. 48V, still has most of its capacity. Reasonable form factor to work with.