A few years ago I was in TH as a tourist but qualified for a Non-O, and had to go to Kuala Lumpur to get it. One of the requirements was that health cert, so I got one from here, while I was in TH, took it to KL, and the Thai embassy there accepted it. Maybe that was a stunt, but I figured I'd try and it worked.
While outside Thailand, to comply for a Non-O, I went to a gov clinic and explained this was a CYA type doc to satisfy an immigration bureaucracy outside the country and that they would never be responsible because the TH gov does not double check the health status of the applicant. Got it for free, signed and stamped. A few years later, needed one of those again to renew my Thai driver's lic. Went to a generic clinic where the counter lady took my blood pressure (sign of life?), and gave me the above cert, signed and stamped. Cost was 200 THB, <$6 US. Good luck!