Everything I've read on the subject seems to be along these lines -- there may be other motor oils that meet the 507 spec but they haven't been submitted to VW for certification. Likewise with the MB spec.
Since there aren't many VW diesels on the road in the US, there isn't much reason to spend the money to gain the certification except for the two biggies (Mobil and Castrol) who sell worldwide.
I can find the Motul stuff at a couple local auto parts stores, but it's under the counter in a glass case like it's some kind of top-shelf scotch or something. I think it's like $12/liter. Never bought it because the Castrol LL03 is so cheap at the dealer.