At a writer's group I recently joined, we've discussed locating and buying used and out-of-print books, partly to find books on writing (which Writer's Digest seems to have the market on), and partly for books in general. As a long-term reader of both fiction and non-fiction I used to become frustrated to see a reference to a book, yet not been able to locate it in a local bookstore or library. But something happened about a decade ago. That Thing is The Internet. From there I located many sources for books of all kinds, and I've yet to be unable to find and buy just about any book I want.
First, the basic sites:
This is of course the big site for buying new books online, as well as the more recent used books (printed after ISBN's became popular and near-universal on books, circa the 1970's), sold through Amazon by its many third-party sellers (I've been one). Amazon has recently been encouraging sellers to list pre-ISBN inventory as well, so older books may be findable on Amazon.
Bookfinder and AddALL are book "metasearch" engines - they look through the databases of most of the largest online book sellers and databases, and show the price and location of perhaps most copies of a title listed online. These include Amazon.com, and related sites such as amazon.ca, so you can look here instead of on Amazon. They both check approximately the same databases, so I don't think one is substantially better than the other (and there are still other "book metasearch" engines out there), but I've used bookfinder since it was started as a college project as mxbf.com (it first worked like a dancing bear (badly), but there was nothing else like it at the time) and have stuck with it through the years.
I've heard of similar sites, but this is the one I signed up for. It's perfectly "free", you can order a book from anyone else out of over a million books listed, but you have you have some of your books listed online as well, and when someone orders one you have to mail it to them (by USPS Media Mail, currently about $2.15 for most books) within a few days. You can list/swap hardbacks too, though shipping may cost more. The currently-selling popular books are generally unavailable here, but once sales wane, copies get listed, so you can always find "last year's" and older best-sellers here. This is just about the cheapest way to get books online. I've been signed up for a little over a year and have swapped over a dozen books.
Online book selling and buying mailing lists.
There are several such, but I've been on this one for many years:
For any book that cannot be located for sale in the above databases, or with a simple Google search, you can post a "WTB" or Want-To-Buy on that list, and many booksellers will see it and search their shelves (which may have many books they have not yet listed online) for the title, and if they have it, they'll quote their price for you in an email. If it's a collectible, desirable or rare book (which it almost certainly will be if there are no copies listed online!), the price may be high, but you will have located a copy of the book you've been looking for, and get to decide if it's worth the price.
Other (offline) sources:
Your local library is good for both borrowing books (usually the more popular and/or more recently published titles, as well as classics) and for buying through library sales (generally including both library books withdrawn from circulation and donated books). I often check the local library's online catalog for a book, and if they have it I'll THEN go to the library and check it out, read it, and then decide if I want to buy it elsewhere.
Some libraries have ongoing 'sales' of a few dozen books by the checkout counter, while others have large sales with up to hundreds of thousands of books, held once or twice a year. These are generally sold at $0.25 to $1 each. I've actually bought books this way that I had previously borrowed from the library. Here's a site listing the larger library and other used book sales:
Yard Sales/Garage Sales
These a scavenger hunt, very hit-and-miss as some sales have no or very few books for sale, but if you're already going to yard sales, it's always worth it to look over the titles for something interesting. Prices are much like library sales and thrift stores, as little as a quarter for MMPB to a dollar and sometimes more.
This is where I've discovered a huge number of books I "didn't know I wanted/needed" (like ANY book on certain subjects, such as writing, because I always knew I 'someday' wanted to 'be a writer') until I saw them on the shelf. Thrift stores generally have a reasonably large selection, usually hundreds and often thousands of books to choose from. There is everything from the past couple of decades, from textbooks to popular sellers, often as recently as within a couple of months of the book first being sold as new. Some stores sort books by category, some don't, but I almost always look over every title (I've learned to scan titles fast), as an interesting book may be misplaced.
Thrift store book prices have been going up over the years. A decade ago most hardbacks were $1, but now they are usually $2 or more. Goodwill is a huge chain that has raised the prices over the years. Their current price for paperbacks is $1.50 (even for MMPB's!) and $2.50 for hardbacks. But these are still close to the best deals available on used books. Thift stores generally have a bit of a funky smell from all the old clothes and such. It's usually noticable (every odor is noticable after 15 years of not smoking) but not a bother, but sometimes these places smell really bad, and staying in such a place for very long is surely detrimental to one's breathing and overall health. There are a few places I've quickly exited and haven't been back to - it isn't worth risking my health to find the one collectible title they might have.
Thrift stores are where I've purchased many books which I've sold on amazon.com (thus funding purchases of some high-dollar hard-to-find books as well as new books), however the bottom continues to fall out of the used book market, with more and more used titles listed on Amazon starting at one cent, including titles I've sold a couple years ago for $5 or more. There are "too many" people like me selling on Amazon, and most regrettably, many of them are willing to sell for a lot less than I am, so I've quit listing books for sale online. The used book market is getting better and better for buyers (well, readers anyway), but worse for sellers. If you're looking for "reading copies" there are plenty, but you want a certain edition in a certain condition, you may have a hard time getting what you asked for, even when a "book dealer" says the book is in "fine" condition. I've mistakenly listed a book as the wrong edition a couple of times, but whenever a buyer has pointed this out I've always given a refund (of the $5 or more purchase price, not one cent!).
Perhaps the biggest thrift store chain (at least based on the many stores around Atlanta) is Goodwill:
In the Metro Atlanta area there is also this local chain as a good source of books:
There are many others, such as St. Vincent De Paul stores, but they usually have a smaller selection. The Salvation Army stores I've seen have a reasonable selection, but their prices are even higher than Goodwill.
Find thrift stores near you at this page: