They sell them on amazon. I bought one a few months ago. It didn’t work very well, so I ended up returning it. It’s such a good concept, but I gave up trying after my first purchase was so disappointing.

It was this one:

Graphene Waist Heating Pad Lower Back Heating Heat Patches Wraps Belt Wrap with Adjustable Strap Contoured for Back Pain Relief https://www.amazon.com/dp/B083YXSRRH/ref=cm_sw_r_cp_api_i_uG8xFbGCAJHNZ