Maybe San Gabriel Nursery. Think they need a cross pollinator? If you get a tree, send me scion when you prune. ;-)

Here is a link that might be useful: http://www.yelp.com/biz/san-gabriel-nursery-and-florist-san-gabriel

Try Sakioka nursery in Fountain Valley, CA

Are some ume trees pink and others white? I've seen photos of both, but am not sure if it is in error.

( From my limited knowledge)

White is normally sakura trees but ume is a light pink sometimes. check the shape of the flowers. Thats your best bet wynnho.

Ume/ plum flowers ....

sakura/ cherry flowers...