look for the pharmacy "Farmacias de Ahorro" nationwide pharmacy chain, offering PCR and antigen tests for usually $300 - $350mxn - about $15 to $20usd. Results in 20 minutes and accepted for travel to the US. Also try Laboratorios Galindo (no idea what they are charging). Since OAX is an international airport with flights to the US you might check with them if they are offering the tests; if yes, they will probably be a little more expensive than at a local pharmacy.