Hello. This is my first post over here--I'm a long-time denizen of the REHP at TMF.
For Mexico, try the website www.mexconnect.com
. It's a pay site now, with some free info and articles. I used to read it a lot when it was all free.
It has tons of info on Mexico and its own set of message boards, although I can't promise that they haven't dwindled since the fee was instituted. There are email addresses of expats living all over the country who will be happy to share information.
Cost now is $5/month or $30/yr. If I were serious about moving there, I would sign up right away for at least a month and start browsing.
I've made 12 trips to Latin America and seen lots of places for rent that weren't overpriced vacation rentals (including in La Paz). But I don't know how you'd locate them from the U.S. Often there's just a sign in the window or a local real estate outfit that doesn't have a website is handling it.