If you haven't already been, I'd recommend t'Arendsnest at 90 Herrengracht as they only sell Dutch beers.
Code:
http://www.arendsnest.nl/
My current favourites are the Klein Duimpje Porters, they have a standard porter, a honey porter and a hazelnut porter. As I always drive to Amsterdam, and take a big trailer with me, I can bring back enough to keep me going until my next trip :-)