Camino de Santi... (not verified) said:

If staying on the south side of town, this big department store with a HUGE supermarket in the basement (C. Fray Luis de León, 29) could be a helpful resource. Open 9:30 am to 9:30 pm Mon thru Sat; 11 am to 9 pm Sun. In addition to the goods to be expected at a major department store, it has a decent hiking gear section on the 6th floor. The supermarket has everything.