You can get most anything you need in the stores in the larger towns, though sometimes its worth bringing it from home because it is a lot more expensive down there. Example: a camcorder tape that costs you $4 in the USA might cost you $12-16 down there.... IF you can find one.
We also brought with us a big bag of PowerBar protein bars. Most of the Mayan ruins sites do not have concession stands on the premises (except Chichén Itzá), though the bigger ones have some outside at the admissions gate. After hiking around for a few hours, you may get hungry and won't feel like trekking all the way back to the exterior of the site. A protein bar will usually tide you over until lunch (or dinner), doesn't take up much room, won't melt, and is always ready to eat. It also has the advantage of being safe to eat (if you're worried about the sanitary conditions for the local food). We brought two dozen bars on the trip, which was almost enough for one a day for the two of us. Special note: make sure you pocket the wrapper when you're done. It is really sad to see modern litter at an ancient site!