I would use plastics at less than 7000m :)   or good insulated leather/ gortex boots. I imagine guiding companies in  

Something burly i think, there are a few mountaineering boots on trademe atm but nothing size 12-13 

Bivouac only rents hiking boots i think, nothing solid enough for Aconcagua.

I would think renting fomr guiding companies over there would be very possible though? best to email ahead