try this site:
http://www.vpracing.com/Poster_Archives/poster_archives.html
There is also the book: "Porsche Plakate, Posters, Affiches" by Jurgen Lewandowski, but it is not easy to find.
I picked up a Sebring '71 and an Osterreich Ring '71 and can tell you that framing them properly will cost nearly as much as some of the posters. I used metal frames, acid free foam core, and uv resistant plexiglass. Since the posters are 30x40 inches the metal frames hold up better, the plexi does not flex across the large opening, and also does not require a spacer, as it "breathes."
Nick