IGN 1:100,000 Chivay (sheet 32-s), Huambo (sheet 32-r) & Orcopampa (sheet 31-r) cover the hike, but inaccurately.
We did not buy 3 topo maps. The simple map in Lonely Planet Trekking in the Central Andes got us through with only a little bit of route-finding.
One trekking guide in Arequipa suggested we photocopy the 3 topos, then cut & tape them together. We didnt bother.
If you insist on buying the topos, try first at South American Explorers Club Cuzco or Lima (Miraflores). If they do not have them, next try in Arequipa at the Colca Trek agency.
More information and a listing of on-line maps on our Map page.