My go-to source for out-of-print books is http://www.bookfinder.com/ It searches all the book stores so you don't have to look at all of them yourself. You can set it to report the postage to your location as well as the price, which comes in handy since sometimes postage cost can be a deal breaker. (But beware if it reports postage as 0.00; this may mean the bookseller's site isn't communicating the postage with Bookfinder.) If you are on the prowl for a certain book, check back often. A bargain may pop up and be snatched in a few hours. Been there, done that, as both the buyer and the one who hesitated too long. It's good for in-print books too. I was looking at a knitting book yesterday. Retail price is $25, Interweave has it on sale around $6, but with a hefty additional $6 for postage. Bookfinder reveals a new copy for $8.00 including postage.
What Bookfinder doesn't search is ebay, which is my other secret. But don't stick with just regular American ebay. Go to the ebay for other countries. I've gotten some rare vintage books, like Endrucks' "Schiffchen-Spitzen", from German-ebay: http://www.ebay.de/ . British-ebay http://www.ebay.co.uk/ is where I've more than once found Mary Konior books and others at bargain prices. Right now, British-ebay has not one, but two copies of the very rare Elgiva Nicholls "A New Look in Tatting." ** I've seen that one sell as high as $200. One is an auction offered by our own Jane, and the other is a buy-it-now. Both cost more than the copy I got last week from Bookfinder, snicker.
**Later: someone has snapped up the buy-it-now copy, so only Jane's is left. Happy bidding!