I had the same talk recently with Papaya Tree Nursery about the waiting list for Green Sapote. Then I did a dumb thing and ordered a grafted Whitman from TopTropicals and another from Plantogram (they call the cultivar "Miami"). I say dumb because both are grafted on Mamey rootstock, which may doom them here in northern California. Should have waited to find trees grafted onto Green Sapote rootstock.
I would up ordering a 3ft seedling from Polynesian Produce Stand on eBay, and also some seeds from Maui Seeds. I'll try to graft my Plantogram/TopTropicals trees onto the seedlings if I can keep them alive long enough.