Lissen up folks!
You're really not going to get much better than this Las Palmas location.
The prices are extremely good, the taco stand is awesome, and they have a full deli. I am not aware of any other full service grocerias in Pittsburgh. The selection is quite complete - you can get anything from a selection of dried chilis to Salvadorian crema, and the produce is well priced as well!
There's even a full cooler if you want to buy some agua con gas or Jarritos!
When I need to find real ingredients (they also regularly carry fresh cactus paddles..) this is the true stop in da burgh.