The Kunsthal art exhibition center in Rotterdam had an exhibition of Alphonse Mucha in 2004.
At the time of the exhibtion the Kunsthal sold a catalog and a big poster.
Perhaps they still have these in stock.
Kunsthal Rotterdam
Museumpark, Westzeedijk 341, 3015 AA Rotterdam
Postadres: Postbus 23077, 3001 KB Rotterdam
telephone: 010 44 00 301
Also the museumshop has some Alphonse Mucha products:
http://www.museumshop.nl/
De Slegte bookshop in the Kalverstraat Amsterdam used to sell antique books and posters on the top floor.
I Haven't been there lately though.