I stopped using AbeBooks when I stopped using Amazon. ThriftBooks and BetterWorldBooks are good alternatives unless you are on AbeBooks for antiquarian books.
Woah, that's cool. I'm definitely using this site. How can ‘they afford the free shipping?
Edit: And they ship free to Canada... no "free shipping" thing ever works to Canada unless you spend a lot of money.
I buy a lot of antiquarian books, many of which are not just out of print, but in the public domain. YMMV.
Local bookstores can typically order virtually anything.
This. Unless you're in the mindset that you need to buy something and you need to have it delivered now (why?), a local bookstore does the job. I typically order books on my closest one, all it takes is an e-mail or gasp I pass by and tell them what I want in person. Never failed.