Any of the shops in the main pedestrian mall (Sarmiento) should be able to help you, though they might not have gear themselves (but I've seen ski gear in one of them). They also stock the purple guide book 'Mountains of Light' for the Cordon del Plata, which is area just outside town with easy high peaks and Vallecitos ski field.
There is (was?) a more ski-oriented shop over on Ave Las Heras, where the big supermarket is. If you leave the main avenue San Martin and walk up Las Heras toward the supermarket and rail line you will see a couple of shops on the right.
The other main shop, Orviz, is along ave Juan B Justo, which is the continuation of Las Heras when it crosses the rail line.
I climbed Volcan Maipo (5230m) last February and it is apparently a popular ski peak in winter, with easy access from Mendoza. A friend just said the Andes have had a snowy winter, so there might still be good cover on it during October.