I think @PlumHill would have scions or suggestions. You might see this thread A Yellow Plum for other members who might have suggestions. You might read this article www.sfgate.com/homeandgarden/amp/Experts-grow-rare-varieties-of-stone-fruits-3202860.php about the gentleman that own this website http://www.arboreumco.com/store/. I did not see the plum listed but I suspect it was at one time or came directly from California rare fruit growers https://crfg.org