In case the Montana stops don’t work for you for any reason, I used to spend a fair amount of time near the Seattle area.
30 miles east of Seattle, about 1/2 mile north of I-90 in Issaquah is a Pacific Fabrics that has a fairly nice upper end yarn and supplies selection, although I doubt they have a staff that is that knowledgable about the yarns.
Should be no problem for a big rig as the road is a 4 lane road and they share a huge parking area surrounded on 3 sides by strip malls and a Fred Meyers. While the area can be busy, the outer edges usually have plenty of parking room even for a big rig.
If you take the first Issaquah exit when going west on I-90 (Front Street/East Lake Sammamish) and turn right at the bottom of the exit. The shopping area will be a couple lights up on the right. You will see a Krispy Creme and Home Depot in the lot before. The Pacific Fabrics is down at the far North end past the Fred Meyers.
Issaquah Pacific Fabrics and Crafts 6000 E. Lake Sammamish Parkway S.E. Issaquah, WA 98029 (425) 392-6696
The next exit west has a Michaels right near I-90. It’ in another area of stores in strip malls. The parking lot isn’t quiet as spacious, but still fairly roomy. If it happens to be too full, there is a Lowe’s just behind the Michaels, and they have a large parking lot, as well as being at the end of the road that goes in behind them, with some wide shoulders that few people drive past.
This link will take you to a map for the natural foods grocery that is in the same parking lot, just across from Michaels. The grocery has a great hot/cold deli and you can sit down for a quality good hot meal for no more than the cost of McDonalds.
Enjoy the drive!