I ordered a white one from evergreen gardenworks back in February. If I recall correctly the website claimed that they would be ready for shipment in May? Mine didn't ship until August, I think there was a random act of God this year that damaged a lot of their material unfortunately.
Other than that I've seen a seller sell pink ones on ebay occasionally, there's a thread somewhere on these forums about it:
https://www.bonsainut.com/threads/impulse-mume-purchase.47816/
And then
@Pitoon has been experimenting with propagation methods. In the future I can see him being a main source.
https://www.bonsainut.com/threads/for-the-love-of-prunus-mume.51086/
Lastly, look into Prunus Americana and Prunus Mexicana. They are the native equivalents and share a lot of qualities with Prunus Mume.