You only need to purchase a Tourist Visa if you are traveling into the mainland (interior) of mexico (anything other than Baja California).
In order to buy a Tourist Visa you must bring proof of two of the following:
Where do I buy a Tourist Visa?
You must first purchase a Tourist Visa application at any Mexican Bank for a fee of $15 per person. Then you are required to go to any Mexican Customs Office(aduana) to start the registration process. After your application has been processed and approved the Mexican Customs Office(aduana) will issue you the Tourist Visa.