They're called maesil plums in Korean, ume in Japanese, I think. I really want to pickle some in a Korean-style I tasted in Jeonju. I've also heard there are Middle Eastern fuzzy green plums that are similar. I know they're not at Han Ah Reum in Manhattan or at Mitsuwa in Edgewater, NJ, except in the already pickled form. If anyone has any tips for where I can find them, I'd be so happy!