Anywhere, where there is a decent Mexican food store, meaning a good size Latino population. Probably Reading Terminal Market...what about that place on Butler Pike in Ambler though? Across the street from where the Acme used to be?
maybe do a search of farmer's markets in your area? if you ever come into philly, i get mine at martin's in reading terminal. may not be the most authentic, but damn if it doesn't taste good in jambalaya.