We visited Uxmal on Wednesday, 8/19 on an AutoProgreso tour bus. The building at the entrance contains a restaurant and several gift shops. A vendor in front of the restaurant sells drinks, ice cream and popsicles. (I think a popsicle was $2.) There is also another restaurant just outside the entrance.
All our family members have Polar water bottles anyway, so it wasn't a big deal for us to bring cold beverages from the boat. You will definitely want a beverage on the tour. It is hot and steamy.
Which brings me to my next comment to anyone touring any of the ruins. I strongly recommend wearing a moisture wicking shirt (UnderArmour, Dry-Fit, etc.).