Public toilets in Mexican cities are quite common and generally clean; look for baños or servicios signs. Many have an attendant who'll hand you some toilet paper when you pay them the small fee of 10 pesos or so. Toilets often lack toilet seats, and it's always a good idea to bring your own toilet paper and hand sanitiser. If there's a trash can provided, place your toilet paper in there, rather than in the toilet bowl to avoid clogging up the antiquated plumbing.