I bought a DVC and a WDW window decal like you described (black letters inside a white oval) from one person on e-bay. I put them on my back windshield. I love to watch people at stoplights try to figure out what DVC means!!
Back in December they were handing out free window decals as members departed the weekly Member Meeting at SSR. They were the static cling type...white lettering with "Disney Vacation Club Member" and the mountain logo.
I was thrilled to get one for free. We looked in the gift shop and at that time it was one of the few things we couldn't find for sale.
We got the same one as tjkraz at one of the DVC Member weekly meetings. I really like it best because it's clear with white lettering and is subtle and very transparent. I bought one of the colored logos at a resort, but ended up using the free one because I like it best.