Found an inexpensive source for these and 10% off for signing up to get newsletter. These will do just fine for pre-heating cooler and FTCing as well. I ordered 2 for less than $12 including shipping.
https://www.shopworldkitchen.com/index.asp?pageID=230&folderID=98
I couldn't really tell much from the description, and I don't mean to sound negative in any way. But from what I've learned from a bunch of very wise people on this forum, when using the FTC method you shoot for maintaining a temp instead of raising it. If these packs come in direct contact with the meat (or foil), you may still be raising the temps and overcooking the meat. If just placing them in the cooler over or under the towels, they could work out fine. I've always found that filling a cooler full of very hot water is a fast and easy way to preheat. Anyway, just my 2 cents worth, I wish you a warm welcome to the forum and hope you have a very Merry Christmas!
I rarely FTC but I could see how these would be convenient to preheat the cooler. Just remove them prior to placing the meat in.
For my first smoke yesterday, the cooler was VERY cold from being out in the garage so I just soaked a kitchen towel with water and microwaved it for 2 minutes then threw it in the bottom and closed the lid. Quick, easy and I just hung dried the towel once it did its job!