Can someone tell me what websites I can use to purchase plane tickets within S. America?

You could try www.lanchile.com - doesn't cover all combinations. It's a OneWorld partner, if that sort of thing is important.

Cheers.

or http://www.sw.com.ar/en/home.jsp
- Southern Winds

Aerolineas Argentinas -
http://www.aerolineas.com.ar/index.asp?idi=es

My husband and I purchased our tickets from Expedia before we left the US for flights from Santiago to Argentina to Brazil. We got great prices at only US$236 per person. These same flights would have cost us US$400 per person if we had purchased them from a travel agent in the States. They are with Aerolineas Argentinas.

Now there are new budget airlines in Argentina and Uruguay. I do not remember the names but they do exist and are very good.