Answer: According to Michael Dirr's "Manual of Woody Landscape Plants" these are actually two different plants. There is a Spirea thunbergii (Thunberg Spirea) and also a similar but slightly larger plant, Spirea x arguta (Garland Spirea).
Unfortunately, I have been unable to locate a commercial source for either one. I suspect there is not much demand for them because so many people prefer the smaller modern varieties and the summer blooming varieties.
Although you might be lucky enough, as I was, to come across one of these great plants at a garden club plant sale or a plant swap, this is pretty unlikely.
Instead, you might want to propagate your own. Spireas are relatively easy to root from softwood cuttings taken in June or July, and can also be layered successfully. You might also be able to dig out a small root division from the perimeter of an established plant -- since you have access to an existing plant of the type you want, this might be the most practical thing to do and very early spring preferably while the plant is still dormant would be the time to do that.
You were right in your guess by the way, these plants were introduced in the 1800's and are native to China and Japan.
I'm really sorry I couldn't locate a source for you.
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