Find a well-rooted sucker at least several feet tall that has enough roots to survive on its own, dig it up with as many roots intact as you can, transplant and water well until established.

Wild/seedling plums grow all over the place around here. I dig them up in the winter when they're dormant. I replant and topwork them. Here is one I did about 3 years ago, it was about 5/8" caliper when I replanted it. Now it's about 3"caliper about 10' tall and has around 20 grafts on it.