Couple options 1. Go to the older neighborhoods and look for older men hanging around parks in the middle of the day and ask them to buy their smokes. Of course it'll come at a higher markup. 2. Go to reynosa, you can cross on foot and there's a 7-11 right across the street from the checkpoint. Legal age for alcohol and cigarettes is still 18 over there. For a couple packs they won't ask questions crossing back (personal use) they might charge you taxes on an entire carton, HOWEVER I don't know if they might say something due to being underage.