Last year was our first year of homeschooling. I bought a complete curriculum set from a friend for half of the original price. After a year with the box curriculum, I figured out the parts we liked, the parts we didn't like, and the parts I felt needed to supplemented. This year, I have used many different sources to find used curriculum. Here's where I've looked for used curriculum:
1) My first choice is the closest used Christian bookstore. I trade in books we don't want, and walk out with the used curriculum we need. Our store even has 40% off twice a year. This is where I've bought most of our curriculum. Of course, this only works if you have a bookstore near you.
2) Homeschoolclassifieds - I bought by son's math workbook here from an individual seller for $12 including shipping. I was going to pay the publisher's website $15 plus shipping. It arrived in less than a week in perfect condition.
3) Amazon.com- I bought a few odds and ends that we needed from amazon. They usually have the lowest price for new workbooks.
4) Cragislist - I haven't actually bought anything here, but I check it from time to time. I have seen homeschool curriculum for sale. The good thing about cragislist is that you might be able to avoid shipping costs. The downside is the time it takes to find what you need.
5) AbeBooks.com - My husband and I bought and sold many of our college textbooks from this site. I've found some homeschool curriculum here also.
6) Trade and borrow from friends
7) Mardel - If you have a Mardel bookstore near you, go visit it! You'll find more than you need. If you don't have a Mardel near you, visit online. I love to shop online, but sometimes, I need to hold the books in my hand and flip through the pages to see if it's really what I want. I could spend hours in this store!
Where do you get your homeschool curriculum?