You can find Mitoku umeboshi products from a mail order company Natural imports. Click on ume to find the choices of ume products. naturalimportscompany.comAlso try the Kushi Institute store or Gold Mine Natural.Hope this helps.
I have found all three at Whole Foods. All in the Asian section. I must say it took me several trips to spot the plums. They were near the top shelf next to the wasabi. The vinegar was on the bottom shelf.