Your best shot would be an online bookstore. Amazon.com has a good selection, often at bargain prices, although shipping costs might be high. Just go to the main page and search for origami, and choose wisely...
Amazon.uk also has 2nd hand books... that's how I got my copy of Harbin Origami 4. I believe that BOS and Origami USA will both ship books. And I think Sasugabooks will also.
So the next questions is, what books are you interested ? Or what kind of suggestions would you like ?
Also secondhand bookstores often have arts/crafts sections... the selection is often sparse, though. Still, if you don't have many books it's a good place to start. Just don't waste money on a really crappy book there are lots of them in secondhand stores because nobody wants them!
Because most origami instructions, I get from videos like YouYube, I now own a couple of origami books and have a folder with about 22 origami diagrams. I'm now looking for instructions on how to make Bahamut (Video) and Bahamut Zero.
It has been said before, but I can't give Nicolas high enough praise for the quality of his service and the products he offers. You will not be disappointed with anything you order from origami-shop.com.