My inlaws live in Mexico so we travel a lot with baby in tow.
There are HEB and Walmart stores throughout Mexico where you can get familiar brands (and Walmart is as plentiful there as it is here). BUT, you WILL pay double or more so be prepared for sticker shock - there is no saving money in Mexico in this department! The two stores I mentioned should have Gerber and Huggies/Pampers...they always have when I go. The labels will be in spanish so just translate your baby's favorites before you go. It will make it easier. Some flavors they will have, some will be different (like mango and papaya). Don't forget to include yogurt.
Car seats are OUTRAGEOUSLY priced. I bought a very basic one there - $85. When we got back to the US, found the same one in Toys'R'Us - $35. So, either rent from the car rental place or haul yours along - worth it. (The one for $85 was the only one for toddlers, dusty and hidden in the corner out of the box - this was at Walmart.)
Bottled water is plentiful. If you use a specialized formula (like lactosse free), I'd haul it. Otherwise, if it's Lipil or some other general brand, they should have it (especially Walmart).
No "queso fresco" or "leche quemada" for baby - often it's unprocessed. Fine for you (but you may get the runs - nothing bad though) but can be rough on the baby.
Find the private hospitals before you go and write down the names. In case of emergency, you want to go there....not public (hotel should have this info). Personal experience on this one. Major private hospitals are almost identical to practice as the ones here. Public are hit or miss at best. Some good, some scary.
Hope that helps! Mexico is beautiful and the people are the nicest you could meet. I'm sure you will have a great trip!