Class C (20mA trip rating) GFCI receptacle/breaker?
The reason you can't find it as a receptacle is that there is no legal use for them in North America. Class C GFCIs can only be used where there is a reliable grounding connection, but that precludes their use where you want to plug something into a receptacle, because there is no way to ensure you are not plugging in a 2 wire device.

What you might be able to do is to put a Class C GFCI breaker on that circuit and hard wire the heating mat. But that is assuming 1) there is nothing else on that same circuit and 2) the heating mat is 3 wire, not 2 wire. If it's only 2 wire you cannot use a Class C GFCI.