I live and fast in Mexico... but know of no specific clinics. I know there are all sorts of health spas around the country, but none that I am aware of focus on fasting...
I have done extended cleansing fasts over the past five years, proudly attaining my main goal a year ago, turning 50 with a clean pink colon (a major achievement in my case)! I have posted extensively on this forum, and even more extensively on the Master Cleanse Forum , and invite you to browse my posts on both forums if details of long fasts interests you. Just click at the top of any of my posts where it says in blue All Somers Posts and voila! Graze away!
All my long fasts have been supervised by my excellent naturopath, who is herself a veteran faster. I do not recommend anyone do extended fasts without professional support, and have been very glad on many occasions that I had this kind of input.
Summer before last I ate a cleansing diet (fruits and vegetables) for 30 days, then went 48 days on The Master Cleanse (a mono juice fast). On day 45 I experienced True Hunger for the first time! When I switched to fresh juices on day 49 I continued to day 60 feeling EXCELLENT! This was really the turning point in my colon cleansing odyssey.
Let me know if I can help you in any way along the cleansing trail.