Envirotex is food-safe for indirect contact only, which makes it suitable for counter tops and tables, but the company will not recommend it's use in food containers. If you are making mugs to sell, I would not recommend using it. That being said, it will work, and is readily available at Hobby Lobby, Menards, and several websites.
My own decade-old leather jack is lined with Envirotex, and after about 5 or so years of use with everything from coffee to rum and coke, the lining started to abraid and become "foggy".
The only epoxy I have found that is safe for direct contact with food, is chemically resistant, and can withstand high temps is a product called Max CLR, and is easily available on ebay (probably other places as well). I contacted the supplier, and they said that by the specs, it should work, but of course they would not give me a completely unqualified answer due to reasons mentioned in an earlier post. I have some sitting in the shop, but haven't had a chance to try it yet. I'll be sure to post the results when I do.
Envirotex is brought up in every thread of this kind that I have ever read, and it's food-safe properties are almost becoming urban legend. While it can be used with minimal risk, it should not be called food-safe.