If you have a list of titles you’re looking for, you can search for them online on Amazon, Abe Books, Alibris, or Half.com. You can also look for them if you join Paperback Swap. It’s free to join and trade books, but you have to start off by posting at least 10 books (paperbacks or hardcovers) that you will agree to mail to someone who requests them.
One of my favorite places to look for used curriculum is Homeschool Classifieds. Their site is http://www.homeschoolclassifieds.com. You can also post a “wanted” for items you would like to purchase. Of course, if you have a good public library nearby, that can be a good source for materials, especially if they offer interlibrary loan.
HTH,
Sue