They have jarred ones at the El Presidente Supermarkets. I haven't seen fresh ones -- BUT, if they exist they're most likely to be at the produce market at Bargain Town Flea Market on South Dixie Highway, south of the Cutler Ridge Mall.
Oh and The Pupusa Factory on Flagler at 19th makes a pretty good pupusa.
Bargain Town Flea Market
24400 Packinghouse Rd, Homestead, FL