Lucky you! They have a great site for natives right in King County.

Here is a link that might be useful: Kingco Natives

For Eastern North America you would probably want Rubus odoratus instead. Maybe our Western North American R. parviflorus can't be grown well, if at all back there - explaining why you would be looking for it here, instead of in Michigan. Another thing to think about is ecotypes (regional variations), for Michigan you would surely have to plant stock from the coldest possible area within the natural range of R. parviflorus to have a possibility of success. Plants grown from material gathered in/near heavily populated coastal lowland areas out here likely to be less hardy.

Plus it requires over a hundred plants to get a significant quantity of berries.

I picked thimbleberries for several hours on 2 August in southern Washington and ended up with about 2 quarts of juice. I picked off of thousands of plants on about 2 miles of roadside.

But even a handful of thimbleberries makes this plant worthwhile.

Raintree Nursery carries/sells them and it's a fun place to visit. While not exactly in Seattle, or even the Seattle area (depending on who you ask), it IS in Western Washington and might be close enough for you to visit.

Either way, they do sell them via mail order too.
Have a great visit and let us know what you do, plant-wise.

Best wishes,
Grant

Here is a link that might be useful: Thimbleberry from Raintree

Visitors from Back East (anything east of Nevada) do not recognize what we call thimbleberry. Thimbleberry is a different plant back east so the tasty jam you're thinking of may not be our thimbleberry. And unlikely anything native to western WA would like Michigan winters.

That said, the Raintree catalog is your best bet, they sell mail order all over the country, my aunt in NY buys from them. They're down in between Mt Rainer and St Helens.

Thimble Berry around Lake Tahoe here in California is in brilliant fall color right now, and I would suspect that plants from 7000 foot elevation would be hardy enough back east, especially if the roots had snow cover all winter, or were mulched. Quite an attractive plant, but plants from Tahoe have not transplanted well when I've tried them here in the Bay Area; they survive and grow, but don't really thrive...