My husband just bought a huge box of Hotties handwarmers from Costco that my daughter said worked well.
I know my hands froze when I tried to use years-old warmers during a soccer match blizzard once, so I'd venture to say that the warmers become less effective as they age...
I use Hothands-2 made by Heatmax Inc. Dalton GA 1-800-432-8629. They stayed hot enough when I took some small tortoises to my sister 12 hours away. And also when I took a rescued box turtle 20 hours to Montana. So I have used them several times and won't be without them in my house. I open them up and mash them all around and they stay hot for close to 24 hours. I highly recommend them...you can also find them on Amazon.com
Things to keep in mind - direct contact may actually be too hot for your tort. I put them inside of socks or something to insulate. Also, need to keep them dry or they won't work. Most take quite a while to warm up. Some are designed to be exposed to little air, like toe warmers for inside boots... so if they get full exposure to air, they heat too fast and won't last as long.