Where can I find good travel deals online to South America?

check out sites like

lonelyplanet.com
travelzoo.com
shermanstravel.com
hostelworld.com (for hostels)

In my experience the best tour company to try in Peru is Karikuy. Their profits go to community projects and is run by one person who gives you tours and books your flights hotels and all that good stuff, very personal and safe. the website is www.karikuy.org.