You can purchase them also from Matt Ouwinga in late winter. In any case I would advise against making root grafts at this point. It is too late now. I am not worried about the tree receiving the grafts, but rather about the seedlings being used; in other words, they may suffer in the process and not give you best chance for success.
Ideally in a year where the tree will also need to be repotted in early spring. That way it will be much easier to work through the roots and create enough space to fit the roots of the saplings.