Have you checked Frankie's on Oahu? Last I checked with them, they did not seem to be grafting Annonas anymore but worth checking with them. Kauai nursery has atemoya trees listed (
http://www.kauainursery.com/images/fruitbook.pdf); I have never ordered from them but when I checked a couple of years back, they will ship. David Frenz at Birds n' Buds in Hilo (808-987-6455) may be worth a try but I doubt that he ships. Chances are, you will only find oe or two clones at any of these nurseries, not the variety that you see discussed on this group.
If you know how to graft & want to grow up some rootstocks, I believe that Oscar at Fruitlovers has scions available.
John