So, are we talking pillowcases then?
I would imagine that those could be used, but I'd suggest that the pillowcase be altered so as not to look like anything that the store might sell in the way of pillowcases. I think most people up there made their own or maybe purchased some from the stores. The most I saw appeared to be hand made.
I can only speak to Oregon on this matter, but it wouldn't surprise me if CA and WA also had the same policies.