Check with your bank and if your debit card can be used in Mexico, just stop at an atm once you're south of the border. Make sure it's a legit atm, like BBVA or HSBC. Changing currency at your bank here, you get screwed on the exchange rates. Pulling from an atm in whatever country you're visiting you get the straight market rate. Maybe a small out of network atm fee, or not, depending on your bank. My debit card works without fees anywhere in Mexico and Central America.
YMMV