These days the Web is no doubt the best place to look.
I get many of my cards from ebay, and below you will find some of the sellers I buy from:
Where do you buy UNUSUAL Playing Cards?
PJ Madsen's Playing Card Museum
Sadly R. Somerville has now ceased trading from his website, but it is still a good source of information and a great place to see other card images.
In Europe general places to look would be Department Stores, Bookshops and Toyshops.
Often Castles, Palaces and Museums stock unusual cards in their gift shops.
For out of print cards, look at these sellers. Most have a good selection and run a friendly service: