Check out Turistel.com They offer the best road maps in Chile. I just don´t know if they will send you the maps to the US. For Argentina road maps your best option are the "Automovil Club Argentino" maps (the argentinean AAA).
I guess you can buy many road maps from Argentina and Chile in the US just to plan your ski trip.
If you need a reference map check out www.skiingtheandes.com/maps.htm
and click on the high Andes section.