Collector Sites
The best sites for rubber duck collectors is duck Planet. It has a nice well moderated forum were collectors can discuss the hobby. It is owned by Charlotte Lee the owner of the Guinness World Record duckie collection.
Another cool duck site is Duckshow. It is a photo blog about many ducky things. It is run by Colleen one of the coolest duck collectors.
Buying Sites
Pricehot
Oriental Trading Company
ebay