Where are you? I'm in London, and I have just been on the phone to Birkenstock's head office, and got a list of shops that sell them. If you are not in London, their phone number is 0171-603 2644.
If you are in this part of the world, here's the list:
37 Neal Street
Natural Shoe Store, 21 Neal Street
James Taylor, 4 Paddington Street
Didn't catch the name, 28 Wigmore Street
Austin Reed, Regent Street
Liberty Church & Co, Regent Street
I work not far from Wigmore and Paddington Streets, and have never noticed these places (never needed to think about them before, I suppose!) but will go and check them out in a lunch hour in the next few days.
Good luck in your search,