I've purchased books from antique stores, consignment shops, thrift stores, goodwill, and online shops. My local St. Vincent De Paul has the best prices. Several weeks ago I walked out of there spending 15¢ on a Henry James novel. I accidentally throw pennies away all the time. I would have been more careful had I known I could buy a library with all those discarded copper colored coins.
Here are some St Vincent books I've purchased in the last couple trips, priced between 13-60¢ each:
here is part of my collection from antique & consignment stores priced between $3-8 each:
a closer view...
There are some great books to be found, but I will admit it takes some thorough searching. However, it is completely worth it to see how excited my best friend gets when I bring home new books for him to peruse through. About as excited as I get when we buy a new ice cream flavor for the first time. Let's leave the bowl out of it, just give me a spoon.
For Christmas one year I decided to give my best friend a book a day for all the days of December leading up to Christmas. I got the idea when Borders sent an email offering free shipping on books with no minimum purchase...so of course I gave it a try with their "used books" section and it worked! I figured it was a mistake, so I ordered about 30 books within 10 minutes and waited for the apology email "we're sorry, free shipping is not actually offered on your $1.20 book". But to my surprise all the books arrived, and I never got charged for shipping. Total cost $60 and a very Merry Christmas. Sadly Borders is no longer in business, but Barnes and Nobles may have the same kind of selection.
These are also great online sources:
Goodwill Books Online
Book Finder Books may not be as affordable as Goodwill or Abe, but it locates the place to buy it!
If you have a good online source for buying your used books let me know!