If it's crazy expensive I was considering carrying around a sharpie and marking rolls I felt had important photos for development to check exposure and approach, saving the rest until I move back to the states.
I was there 10 years ago and used a lab called LMI who did good work. They were processing C41 and E6 for me. I did a quick google search and it looks like they are still in business so you could ask them. It would be helpful if you speak Spanish as when I was there none of the staff spoke English.
It's a little hard, but I think there are one or two labs in Mexico City that actually still do E-6. LMI is one of them (as pointed out). I THINK Foto Citlali also does this, not sure though. Best of luck.
If you don't mind home processing (E-6 is a bit more toxic, wear gloves) then you can buy the home E-6 kits (may still be too expensive to ship to Mexico). I use a sous vide cooker to keep both that and my C-41 chemicals to temp in a large water bucket.