If you just need a refill on current prescription, Costco. Includes free eye exam, if desired.
http://www.costco.com.mx/view/c/joyas-y-ropa/optica/lentes-de-contacto?showAll=true
Contact lenses in Mexico are about 30% cheaper than in the U.S. (Walgreens, etc.)
They also didn't seem to be too concerned about having a current prescription.
I handed them the old box, and they gave me the same prescription. They occasionally have sales,
some advertised, some only in-store. For more compicated vision issues, Dr. Pinto
http://www.chapala.com/pintooptica.html