We have a Japanese plum which was identified by an arborist a few years back as a Santa Rosa. It bears red-skinned plums that are wonderful for jellies and syrups. It was an old tree when we moved here 28 years ago and we know it won't live forever. We would like to graft scions from our old tree. We saw the article in the local newspaper about the Home Orchard Society's annual Scion Exchange and Rootstock sale at the Canby, OR fairgrounds on March 5th. We are interested in buying some rootstock at the sale, but after finding this website, I see that scions should be harvested in the winter, when dormant. Is it too late to harvest them from our tree now, when their beds are swelling? Should we wait till next year? Can we purchase rootstock now and use them next year, if it is too late to harvest the scions now? And what variety of rootstock should we purchase?

First off, Santa Rosa is not uncommon; in fact you can often find it for sale in those tightly wrapped plastic bundles outside most grocery stores for the next 3 months - cheep. The HOS Rootstock available is "Citation," and "St. Julian GF-655-2;" I'm only familiar with the St. Julian so shouldn't give a recommendation.
There should be a generous amount of Santa Rosa plum scions available at the Scion Exchange, but if you're determined to clone this very tree, act fast. I'd look the tree over for any pencil diameter shoots of "new growth" that haven't begun to "push" yet. They'd be closer to the limb - the further away you get, the more growth hormone has arrived. There are secondary latent buds on either side of the "main" bud. If you find any "tight wooded" shoots, it only takes 2 or 3, cut the entire piece and store it as described by others in your refrigerator. When anyone insists at the grafting table that we "just go for it," we can rub off the pushing buds, make the graft, and assume the secondary buds will do their thing (grow).
Lots of good questions: If you procured rootstock "this year" you'd best plant it out, you could pot it, but I don't see any advantages in that. And if you plant it out, you'll be making the graft yourself next year - a bit trickier on "stone fruit" than apples & pears...
My suggestion... and I don't think anyone's ever done this: buy a "bagged" Santa Rosa plum from Bi-mart / Fred Meyer.., bring it to the Scion Exchange, choose a pollinator scion, bring it to (me) the grafters and have us graft it to a limb of that young tree. - This is simply the way I do it any more, and only a suggestion - but good questions, and good luck!

Back again... after re-reading my "suggestions," I remembered another "trick." You may also "Bud" your plum this summer to a rootstock purchased and planted this spring. Make a decision on the two rootstocks available; either plant it where you'd want the eventual tree, or pot it up for the summer.
I'm not into "Budding," but do understand the process. In July or August (HOS usually puts on a Summer Budding program) you would slice off a fresh living bud from your Santa Rosa plum, make the "T" cut on the rootstock, incert that tiny bud and seal it up!
If the bud does not survive, simply store some scions from your Santa Rosa (earlier) next winter, attend the HOS Grafting Workshop, then go home and (top-work) graft that rootstock again. If that graft didn't take - bud it the following Summer... (Or buy a bagged tree~)
I didn't feel right leaving out the "budding option" - so now that your're totally confused - have even more fun!