You'd have to check with Alison Swope about what form, exactly, she gets her ground corn from Moctec. As you know, fresh masa is very perishable. You can get Mexican corn in 1-pound bags through Gourmetsleuth.com. They are in California. The corn still has the hull on it, so they send you lime as well to remove it. Pretty painless.
Another local chef who's into authentic corn is Susan McCreight Lindeborg at the Majestic Cafe. She gets hers in 50-pound bags from Buena Vista in Albuquerque. (She's from New Mexico). It's a much smaller, almost flint corn grown in the Southwest, the same corn you would find already cooked in a can of Juanita's brand hominy.
Rest assured that Mexico has switched over to American field corn. Maybe some of the big commercial farms. But Mexico has hundreds of strains of its own corn to use and they would not let theirs die out so easily.