I am looking for rootstock to graft hydrangeas.
Do you have a hydrangea now. Then root cuttings. This will give you the root stock. Are you trying to cross breed?
A good way to root hydrangea is to lay a branch on the ground and weight it down with a rock. Or you can just cut a stem and put it right in the ground. Soft sandy soil is preferred but any soil will do. Just keep it moist. You may can get some going before winter. Try it. Also try again in the spring with the new growth.