Wow, any place with forest! They are everywhere! What is actually needed is to train your eye to see what is right in front of you when not in flower. Many are small.
The highest concentration of epiphytes will be in cloud forest, such as that surrounding Xalapa or similar places. If you can get to Cuicatlán, look up in the mountains enclosing the valley. The zone with oaks has orchids covering all the tree branches. Almost any of the river valleys will have Laelia orchids up the slopes in the zone where oaks grow.
There were many terrestrial species in the Valle de México before the city grew. Some may survive in parks that have not been disturbed.
It will be hard to find, but see if you can get a copy of
Orquídeas de México.
And there must be an orchid society in México City?