Our plums bloomed early too and it seems like every single flower set fruit, so I don't think we have a bee shortage here in SC OK. Is there a chance that someone near you is spraying insecticides that is killing the bees? If that is the case, buying and releasing bees of any type may not help any. I always plant cool-season flowers like sweet alyssum, pansies and dianthus that bloom earlier than the plums. They attract the pollinators before the plums ever bloom so that by the time the trees are in bloom, the pollinators are used to visiting daily. You're further north than I am, so I don't know if you could coax the flowers into bloom early enough, but it really seems to help.

I do remember thinking back when the plums were blooming that the bees weren't around, but it was more because it was cloudy and rainy every day. Once we had a sunny day, there were bees everywhere. It might have been the weather if y'all also were constantly cloudy, cool and/or rainy while the wild plums were blooming.