SoCal gal in Cherry Hill needs her masa to make tamales.
Are you looking for just the flour? Don't know about cherry hill but south philly has many small markets that have it, no big markets. The shop rite in the port Richmond section of philly, right off aramingo ave, has a large Latin section and carries masa too. Both much closer than norristown.
It's definitely NOT a larger mercado, but there's a small, family-run Mexican market on Route 70 in Cherry Hill. I don't remember the name, but it's just a couple doors down from their restaurant, La Cita. The restaurant is very casual but we enjoyed it a great deal.