First, there is a specific Mexico/Self Pay forum here that might get more or better answers.
I can't speak specifically for any of the MX practices, though my wife was self pay here in the states, Most follow up care is done by your PCP including routine labs, and those are generally covered by insurance. From what I have seen, most US bariatric practices aren't keen on doing follow up for other doctors (partly not wanting to support the competition, partly not wanting to get involved in someone else's surgery and potential problems) though longer term if revisions are needed, they are more than willing..... I would expect that the better practices would offer some kind of remote follow up (phone/fax/email/skype, etc.) for some time, but don't know for sure on that. The practice that we used here in the states was set up for travelling patients, so that is routine for them to do the periodic post-op follow ups as well as the annuals. One can also often join the support group for local surgeons to get that kind of support - many welcome patients from other practices - and that may also be a way in to see them if there are any problems that need attention (ask questions at the group meeting - I'm having this or that problem - and that may interest them enough to invite you to make an appointment, whereas they may refuse if you just call the office cold.