I found what you are looking for in the Jung Seed Catalog.. page 11. item # is 30441 (this gives you two plants for 37.95, one male and one female). It is necessary to have one of each . Kind of expensive...but in case you cannot get seed from anyone. Looks beautfiul and I may have to buy this for myself! their website is www.jungseed.com.

Abuse them. My Okie summer last year was awful. All the seaberry got was a splash as the hose went past, between the strawberries and the goumi, which needed water all the time. The seaberry got a decent drink when I watered the lawn every 10 days. It took me a while to realize that I have seaberry fruit this year! They are a very inconspicuous thing.

Jane, yours is a wild cherry, Prunus serotina, not buckthorn. It doesn't have the prominent leaf veins on the leaf underside that buckthorn has, and the edges of the leaf are finely toothed unlike buckthorn's smooth leaf edge. The cherries have their own stem with nothing but cherries on them rather than the buckthorn's many small stems for berries off of the main woody stem/branches. If you open one up you will recognize small cherry pits, and there is a bitter cherry taste if you are brave (I think buckthorn has 2 or 3 seeds.) One of the keys is the small lines in the bark (technically lenticels) on the cherries, where the lenticels on the buckthorn are dots. The other key factor for me is when you scratch cherry twig bark it smells like almonds, while the buckthorn has its own distinctive odor that isn't easily described, but definitely isn't almond. My memory is also that the buckthorn fruits have a clear yellowish flesh under the dark skin, while cherry fruits have the dark stain in the flesh in the same way commercial cherries do. Prunus serotina is native, and while it can be messy (the fruit stains as you would expect cherry to do) and it does seed around some, it doesn't IME outcompete and choke out other plants as buckthorn can do. Here is a link that might be useful: Prunus serotina, wild black cherry

Sadly, I agree that it is Rhamnus cathartica. Really tough invasive to get rid of. They laugh at being cut to the ground. I've read a great article (with positive results) on the non-chemical removal means they note in this data sheet: https://files.dnr.state.mn.us/natural_resources/invasives/terrestrialplants/woodyplants/buckthorn_what_you_should_know.pdf