Where to buy pesos

Do a Google search using Piedras Negras Cajero Automaticos (hightlight and right click here) and you will see locations of ATMs in Piedras Negras on a Google Map. There appears to be a Bancomer and a Santander within blocks of the border. ATMs almost always give the best rates. The Google search will turn up others. We prefer to use machines at banks where they usually have the lowest fees and if something goes wrong you can go in the bank to discuss any problems.