Update:
Talked to a number of tire dealers and distributors. They all say they can't even order them. No 13" tires / wheels that will fit due to the diameter.
So the suggestion by moisheh is about the only recourse short of raising the body higher off the frame to allow more clearance. We found out a friend is coming from Tucson in a few days so we can have them bring them if necessary but we decided to go ahead and order some radials from the states for the trailer that are rated 10 ply with about 1,500 lbs max as opposed to the commonly available 6PR 1,050 max load. A bit on the expensive side, but should beat changing tires every 1,000 miles or so.
Moral of the story: Do better research than I did into what tires are available in Mexico before you go! :)
12" trailer tires are NOT AVAILABLE in Mexico.
Thanks all for your suggestions and help - I really appreciate it.