Why exactly are you looking for PR 5 and 6? I had a short talk with Uncle Google about this just now. Apparently, Pinterest is ranked very highly with PR 9. I have looked at the sites with PR 5 and 6 and I couldn't recognize any of them. So if you are thinking of using a site to create backlinks to your site, I would recommend using Pinterest. It's not difficult to post to Pinterest. Just post some images with links to your site.