I just stumbled on a tortilleria (tortilla factory) in Norristown. On Chain Street (I think; maybe it was George Street) just south of Marshall. Tasty tortillas, sold by the pound, hot off the conveyer belt. They also sell uncooked masa. There may be other tortillerias in the Phila. area but this is the first one I've found.
The nearby blocks of Marshall Street have a number of Mexican groceries, restaurants, and other businesses. There's a big sign on the wall of the corner building on Marshall (looking uphill) directing you to the tortilleria.