I've found that most of the best hotels in Latin America aren't part of any big international chain, apart from a few here and there run by Four Seasons, R-C, and Orient-Express. This Luxury Latin America site has done a lot of the weeding out and reviews what look to be the best choices in each country.

I've also found some gems through the LHW site and the Small Luxury Hotels site. Like some others have mentioned, some good deals sometimes on LuxuryLink.com and even SkyAuction.com for some spots. (Just be sure to read all the fine print to tally up the fees.)