Buy other collections:
Some people that might be updating their personal collection will sell off older books that they might not want. Check out other book collection that you might be interested in as you can always buy these books and add them to your home library.
Most libraries around the country usually sell off a good portion of their older books to make room for newer editions and other collections. Be sure to take a few big cardboard boxes when you go to a library book sale as most collections are sold by the inch or by weight. These sales are a good place to go if you want to buy a lot of books to stock up your home library.
Go over your collection and organise them in such a way that you can easily keep track of everything. You will find many books that you might have thought were missing or lost. Keep a system to organise your books in your home library.
Visit garage / yard sales:
You will find a lot of books for sale at very reasonable prices at local garage / yard sales. Most people sell off their old books that they might not want for some reason and you can get a good deal on many different types of books that you can use in your home library. Remember before buying a large amount of books it is a good idea to take a quick look at the different titles as you do not want to buy duplicates of books that you might already have in your library.
Get on the internet and you will find a huge selection of various websites that sell used book collections. Be sure to only purchase books from a reputable website so that your credit card information is kept safe. Also, be sure to factor in the cost of shipping and handling as large collection of books can be quite expensive to deliver.