Just got off the phone with Dave Wilson, was transferred to Sean in sales department, great guy.
Dave Wilson does not sell Prunus mume rootstock. The closest they offer is 'Myro rootstock' (Myrobalan Plum, Myrobalan 29C, Prunus cerasifera), in 3.8" to 1" diameter.
Minimum order is 200 units. Shipments for 2022 are done for the year. ALL of their material for 2023 is already bought/under contract, and nothing will be available unless people cancel orders.
They are also working grating a few cultivars that are fairly common in the USA (Bongo, Peggy Clarke, and Shiro Kaga), which may be available in the future but not in 2022 or 2023.