Alta Baja Market is located inside 4th Street Market (link) on the corner of 4th St + Bush.
From Los Angeles: take the 5 fwy south to Main/Broadway exit (directly after the 22 fwy), take the Main St exit and turn right on Main. Turn left on 5th street then right into the large structure off French.
From San Diego: take the 5 fwy north to 4th St exit and turn left on 4th . Turn right at French into the large structure
Parking: if you are coming in for a quick stop there is metered parking on the street or you can access a large parking lot off 5th + French. All parking is free on Sundays.