The Cordillera Blanca [1] is not the best place for ski mountaineering. Finding good snow often means climbing well above 5,000 meters.
Because of rapid glacier retreat, even the most experienced guides cannot keep track of new crevasses from year to year. That said, if you are interested, the snow is best April–June, and you can hire both guides and reliable ski equipment in Huaraz [2].
The most frequented skiing spot is Glacier Pastoruri, and other good options include moderate mountains such as Huálcan and Copa. A good contact is Alfredo Quintana at MountClimb (Mariscal Caceres 421, tel. 043/42-6060, www.mountclimb.com.pe [3]). Skis can be rented here or from Enrique Bosshard at Inka Pub Monte Rosa (José de la Mar 661, tel. 043/42-1447, pmonterosa [at] yahoo [dot] com, 11 a.m.–midnight).
Links:
[1] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/trekking-the-andes/cordillera-blanca
[2] http://www.moon.com/destinations/peru/huaraz-and-the-cordillera-blanca/huaraz
[3] http://www.mountclimb.com.pe