Another plug for libraries, public or otherwise: most have access to WorldCat, a catalogue of (reputedly) all the contents of all the libraries in the world. Actually, it's a catalogue of most of the contents of lots of libraries. Lots and lots, but not all.
The first great thing about it is that, if you know the title--or the author's name, or something close--or even the title (etc.) of a book on the same subject, you can (a) find a catalog record for that book, then (b) click on the entry in the list of subjects that best fits the one you're after and get a long list of books with the same subject that might well contain the one you're looking for, or something just as good.
The second great thing is that there is also a link from that catalog record for the book you're looking for to a list of all the libraries (among those with contents listed in WorldCat) that own the book.
Third, libraries that have access to WorldCat often participate in an interlibrary loan service that allows libraries (and patrons of same) to borrow books from other libraries. I have gotten things from across the country as well as from cities 20 miles away. The system used to be kind of slow, but it has picked up considerable speed--a week or 10 days is often the time-lag between requesting and receiving items now.
And then, there's the second-hand bookstore. 2/3 of my personal stuff I have purchased for a total of about 1/10 of the original retail price.