I have always found it with the other foot care products.
I will say there are two different types. One kind is "mole FOAM" which is VERY, VERY thick. I made that mistake on our January trip. When I opened it to cut a piece I realized it is like felt, only thicker! I stuck it on the back of my heel but it made my shoe very uncomfortable!
Better to try to find plain old mole skin. If yoiu can't find it they probably have it at First Aid in the parks. I know they had it at AK the day we were there in the pouring rain. There were LOTS of people with blisters from wet shoes & socks!