I was pitching a reality called "sex-pats": They come for the malbec, they stay for the sex. The main problem if finding people willing to be followed by a camera (probably easier for a radio story). I have met some expats for whom sex tourism is the second reason for being here, but few for who it is the first.
As a documentary filmmaker, you are going to have a hard time finding someone to appear on camera. Not impossible, but very hard, especially if they are from the USA where things are more moralistic. You could probably find an Argentine to talk on camera (google Guillermo Coppola).
I would suggest instead of asking on a list serve, go everynight to the boliches where sex is sold in an open manner and make friends with the owners and regulars, after a while you might find someone willing to talk, but you are going to have to work and build trust.