You should check out the Pacific Fabrics Outlet store on 4th ave in Seattle (near the downtown Costco). They've got a huge selection, including all kinds of vinyl/oilcloth/misc fabrics. It has sort of a warehouse feel, and (depending upon what you choose, of course) seems to be cheaper than many of the other (mostly east side) fabric shops.
Pacific Fabrics and Crafts Outlet Store
2230 4th Avenue South
Seattle WA 98134