Yes. Like most countries, in Mexico doctors give advice on what to take, not dispense the permission to buy it. Pharmaceuticals are distributed through pharmacies, and doctors are not gatekeepers. Ivermectin is a very common drug, and most pharmacies will have it in stock. You may have to look up a local brand, but the pharmacist can probably figure it out. You should determine the dosage you want before going to the pharmacy, especially if you don't speak Spanish.