For topographical maps, go to the Instituto Geográfico Nacional. In January, the IGN closes early, so call ahead. Its maps are for sale or for reference on the premises. It has a good road map of Peru (1:2,000,000) is S51, and a four-sheet 1:1,000,000 topographical map of Peru costs S120.
Departmental maps at various scales are available from S19. High-scale topographic maps for trekking are available, though sheets of border areas might be hard to get. Geological and demographic maps and CD-ROMs are also sold.