There may be more available in the near future from the CRFG chapters in your area.
Our chapter has been making new Zill varieties as well as other well-regarded varieties, including the ones known to be successful in our area available to the public at our annual plant sale, though there have been other opportunities for those who are active in the chapters.
Typically the focus has not been on larger trees as trees seem to establish themselves better if planted at a younger age. So typically the size available is the #1-#5 nursery pot size with some #7 (5 gals) available.
My recommendation would be to get in touch with your local chapter and convey your interest, you may be surprised at the opportunities in the coming year.
If you're looking to speed up the process, the recommended approach is to start planting seeds at the nearest opportunity. Once seedlings are established, the trees can be topworked into the sought varieties by approach grafting or other techniques, but the sooner a seed gets in the ground, the longer it has to get established.