I arrived in Mendoza without any prior arrangements. I bought the permit and provisions in the well stocked supermarkets and off I went. First to Cordon del Plata for a week of acclimatization. When I felt ready I walked in (no mulas) and climbed the peak.
If well acclimatized, I don't think it's a big deal to carry in the gear needed for the climb. If you run out of provisions, you can always eat in one of the many restaurants and buy food, white gas and gas canisters in BC.
If you want to arrange a deal with a company in Mendoza, pay Hostel Independencia a visit. Aconcagua Xperience has an office there.