Cheers!
http://www.abebooks.com/
because a good number of these are not in print in the U.S.
You can get some of the Swallows and Amazons series in paperback (from Godine) at Amazon, but really abebooks has the best deals on the planet.
I buy all sorts of oddball stuff from them at dirt cheap prices ($5-$10 for hardbacks, a couple of bucks for paper). Like the Amazon used books, you're dealing with individual book shops, but I haven't been stung yet. My best deal so far is a Liber Usualis (Gregorian chants) that retails new for over a hundred bucks -- for $25. It's a little beat up, but it has all its pages and the binding is sound.