Did you make it to Puerto?
As for you question, yes, it is nearly impossible to find purebred chickens in Oaxaca, at least that has been my experience. The local vets will sell "Rhode Island Reds" and "Barred Rocks", but they aren't purebred. Then there are all the fighting cocks and the local mixed breeds, and that is about it. Oh, someone sells Silkies near Tlaxiaco for 100 pesos. And I have some Marans and Easter Eggers.
DO you have Marans there? Cool. Nunca escuche de alguien que tuviese Marans allá.