I'm trying to get 100 this year from:
River Hills Harvest because, to me they seem to be the most reputable.
You could try
https://www.elderberryman.com/ It says that they're organic, but they are very expensive.
The cheapest I've found are from a company that seems super shady, so I don't even want to reference the company on here because I don't think THEY even know what variety they shipped me even though they had different varieties listed on their site.
Although they only provide rooted cuttings of elderberries,
Fruitwood Nursery is a good place for scion/cuttings of basically any sort of fruit species.