I was looking around and found it was really hard to find amla locally (I live in Vancouver, Canada). Anyway, based on its origin, I went to Little India in search of it and found it in the spice section. It's very cheap. I think I paid $2.50 for a good sized bag of amla powder.
They use it in India for cooking too!
Anyway, I hope this helps some people find some amla in their local Indian food store.