I have the Thermotex Platinum Infrared Therapy Series 17" x 15" Inches Heating Pad.
I didn't have an Amazon or Ebay account back when I bought it so didn't price check on those.
But they seem to be about the same price as I paid , but I didn't see the other brands back then, so those might be just as good.

If you need something fairly flexible the Thermotex won't work very well, the size I have is made up of 3 solid bars under the padding. I usually just have mine on the back of the recliner and lean my upper back or lower back against it in the evenings.
Husband uses it on his knee that he hurt this summer.

The other brands seem to have a more flexible look to them. Some of them are larger too.
Amazon has a pretty good amount of reviews on each product so it helps to read those too.