If you are looking to locate near the ancient glacial areas of the PNW, then you will find clay somewhere on the property. I would not base the purchase of the property on just that. You will be importing large quantities of sand and of course straw. Once you add water to make the clay slip for the cob, you really don't need all that much. Low clay soils, as long as there is not too much silt, can be amended with powdered bentonite to up the clay content.
We did not build a cob house, but we did build a round cob bathroom (there's a story to that). We loved the material for building, but we discovered that it is very time-consuming. If you are planning to have that be your work, then fine, build small and you will do well. Sun Ray Kelley who builds (unpermitted, crazy, wonderful) houses has built some pretty amazing, large cob structures using a bulldozer to mix large quantities of cob at the same time. You'll find pictures of his buildings in many books on hand-built homes.
What I loved about the material was it's flexibility, especially for low-skilled homebuilders. You start to put in a window and cob around it, then check if it's plumb. No? Push Push Grunt Grunt Cob Cob--voila!--Perfect! Then you can cob up and around the roof materials. Very elegant in a practical sort of way. The earthen paint spread on top is such a joy to use compared to sticky, smelly house paint. Really a lovely way to build if you have the time, and in many ways more appropriate to the PNW than straw bale.
One more caveat about cob-- being essentially a stone house (made with itty bitty stones) it takes a long time to heat. Granted, it never gets that cold, either, but if you will be spending most of your time at work, unless you keep the home heated it will be cold when you get home and not that warm until morning. If you are there most of the day most days, you'll find it quite comfortable.
My favorite how-too books have been by Ianto Evans, and Becky Bee. It's been a long time since I've researched the information so new and wonderful online links might be out there as welI. I highly recommend you attend a workshop or at least a cob work party to get a feel for what you are doing, mainly with the mixing of the cob. Every locale will be a bit different because the clay used will be a mite bit different, but you will get a good idea as to what a good cob mix looks (and sounds) like. If this is out of the realm of possibility, start with a little cob shed, like our bathroom, and get practice working on that. In fact, do both if you can. You may decide after building your sweet little outbuilding that, as wonderful a material as it is, cob is not right for your needs.