The one and only place that MIGHT have it is LMI (Laboratorio Mexicano
de Imagenes) their phone number is 5515-5540 ( this is the number, not
including are code). I live in Queretaro, 3 hours from Mexico City and
have been unable to find 4x5 let alone 8x10. I import all my film from
badger and photowarehouse, both 8x10 and 12x20.
My recommendation, you should bring your film with you, or have it
sent to your hotel, but if you have it sent, you will have to pay 25%
import taxes.
Good luck and make sure you travel outside Mexico City, it is far
prettier than that cess pool.