Well, people may not know what they are and many are certainly expensive (>$80), but there are a lot of them at your local home center. Some are just "Heat Guard" (160/135F), but most of them are full mixers with the little expanding wax valves.

Here's a page full:

https://www.homedepot.com/b/Plumbing-Valves/Thermostatic/N-5...