Oh! I'd be interested in hearing more about this topic! We grew Japanese plums if the front yard when I was young. Though, my brother and I just ate them (before they were ripe and after!). Unfortunately I don't know what variety we had.
According to This site [url]https://en.wikipedia.org/wiki/Ume[/url] the species of plum you are looking for is Prunus Mume . It seems to be more closely related to an apricot than a true plum.
A lot of the places I looked for a source was more focused on this species as a bonsai specimen. Perhaps asking for a reputable source on the Bonsai forums here would give an idea where to find a larger specimen of this plant?