The largest market in San Pedro [1] is the Mercado Medina-Concepción, so named for the two neighborhoods in the southeast part of the city in which the sprawling market is located. The main market building is between 4 and 5 Avenidas and 6 and 7 Calles SE, but the surrounding streets are jammed with vendors of all variety.
This is not an area all foreigners will feel comfortable walking around, even during the day, and it should definitely be avoided after dark.
Just up 7 Calle at 9 Avenida SE is the smaller and newer Mercado Dandy, built in 1991 to ease the pressure on the main market.
Links:
[1] http://www.moon.com/destinations/honduras/central-honduras/san-pedro-sula