I went to the states 2 years ago and purchased all the seals OE from Buds Benz and had them shipped to me whilst I was staying in Seattle.
When I returned home and some months later, I noticed the push in seals on the rear quarter windows were supplied short and David Latham had no issue in sending me replacements directly to Australia.
I also made a large purchase from Authentic Classics at the same time, so either of those two businesses would be my recommendation.
Doug.