Why do you want Elderberry root stock? Are some elderberries grafted? Sambucus nigra ssp. Caerulea our western blue elderberry that grows
native here in the Missoula area (and North to B.C. and South to Baja) is a pretty delicious elderberry. I have a blue elderberry in my front
yard that Helen Atthowe started and sold me at the Missoula Farmer's market about the year 2000 and a American Elderberry ot Sambucus nigra ssp. Canadensis in the Backyard. Also a newer seedling in the backyard that might be a hybrid that a bird planted.
You could grow your own rootstocks. Though it takes acid stratification.
Unless there is a new Helen at the farmers market who will start them. You might have to order them mail order.
Western Montana gardener and botanist in zone 6a according to 2012 zone update.
Gardening on lakebed sediments with 7 inch silty clay loam topsoil, 7 inch clay accumulation layer underneath, have added sand in places.