I use a 'retired' cotton mattress pad for the innards of potholders...old ones are best, thicker and more closely quilted...and very minimally quilt them. Remember, every time you take a stitch through the layers, you put a hole through the heat shield.
The mattress padding is plenty enough heat protection for anything coming out of an oven (unless you plan to hold it for an hour or so), it's easy to work with, it results in a nice 'grandma' type potholder and it's cheap.
I just can't see buying special batting whose only purpose is to resist heat, has no real safety advantages (it is NOT flame resistant, and does not claim to be...it's almost entirely poly after all), can be difficult to find, and only works if the correct side of the potholder is facing the heat.