I haven't been there yet so I can't clue you in from personal experience, but hearing from guys like GaGambler, you don't use any boards or ad sites down there. The best method is picking them up in bars. GaGambler can give you the insights as to which bars, what time of day, what the going rate should be when you negotiate. (and negotiating is expected, unlike here in the US). You don't want to get taken as a naive gringo like the dumb secret service guys.

GaG can come off as gruff and grouchy on the public boards, but he is a wealth of info on the Latin American countries and is very helpful backchannel.

Good luck and have fun.