So, you've scoured your shelves, begged from your family and run out of books on hand? The time has come to begin book hunting in earnest. There are three primary techniques that I have found useful for finding more stock for your shelves: (1) the Internet, (2) deals with acquaintances, (3) sales and shops.
There may be other options out there--and I'd love to hear about them--but the best places I've found for buying books online are Craigslist and Freecycle. Craigslist is a classified ads service that is free in most parts of the country. It has a dedicated book section which is a bit of a crapshoot but it is possible to find great deals on books, especially when someone is selling off a collection. Freecycle is a posting service where people put up objects they want to give away to the first comer. Books are far rarer on Freecycle than Craigslist, but it offers a convenient daily email service which is worth a look.
Once it becomes widely known that you are interested in books, people who have them but aren't able or willing to sell on their own may approach you. Sometimes these can be very profitable, such as a deal I had with a nonprofit in my town that was liquidating its library, I handled all the sales and we split the proceeds, what I didn't sell, I brought back to them for a rummage sale. I had a similar deal with a friend for whom I sold a collection of Star Wars posters, my single biggest sale to date. Figure out what you think is a fair trade, make sure to account for shipping and other costs and present these partners with an offer. Sometimes it might be worthwhile to let potential partners know what your deal is--either by email, personal conversation or a post on a social media website.
(3) Sales and Shops
This is where the thrill of the chase really kicks in. Thrift stores, garage sales, and library sales are major sources of books and I try to make the rounds once a week to look for stock. Big library book sales can be a bonanza, though they often require long drives, commitment of a day and early mornings (best to get there when they're opening). Check out Book Sale Finder for locations near you. Less dramatic, but also useful are the everyday types of sales many libraries have. These are worth checking out, especially as librarians often presort out worthless books. The best deals come from thrift stores: Goodwill (Store Locator) and the Salvation Army (Store Locator) in the United States; in Britain I found Oxfam Stores to be good stops (Shop Finder). Garage sales can be good sources, though I find that the big estate sales are better than small sales: when someone is selling a handful of objects they tend to price them higher than you want to pay. Books available in big estate sales tend not to be priced with nostalgia in mind, meaning that the prices tend to be lower. Flea markets/Swap Meets/Car Boot Sales can sometimes have good finds, but they tend not to be book-heavy locations.
Good luck and don't forget that when you're buying don't forget the calculation: Profit = Quality - Condition - Cost