I have an uncle who moved to Mexico, and when I send letters to him, they go to a Texas address. He subscribes to a service that then forwards his US mail to him in batches. It takes about three weeks for a letter to get to him.

I doubt that customs is too worried about out-going plant material into Mexico, so that might be an avenue that you could research. I would worry that tubers might get fried in the heat, though. That is a long time for them to be bagged up with no ventilation and protection. Seeds might be a better choice for you, but the beautiful named cultivars only come from tubers.

Mike, there is a Asociacion Mexicana de la Dalia o Acocoxochitl, and they have a "Dahlia Network" called Red de Dahlias. I don't know where this Mexican Dahlia Society is located, but if you can connect up with them, they might have information on how to obtain varieties already grown in Mexico, and any special cultivation issues for growing dahlias in that climate.