There are several places to save that extra buck on books. There's always good 'ole ebay, but you have other options.
Vegsource.com is a great site to buy or sell. You just click on the homeschool button and you are on your way.
Homeschool groups are another. If you are part of your local homeschool group, then you can just ask around. You never know who has what sitting on their shelf. Maybe they plan on using it in the future, but you can always rent or borrow it until they are ready to use it.
Book sales are another. Check out the counties or cities near you. Watch the local newspapers for upcoming sales. Or a used books store may have some as well. Private schools donate a lot of their textbooks to places like used book stores, goodwill and even thrift stores!
Last but not least, check with your local library. You'd be surprised on what curriculum they have. Most libraries have a budget for purchasing new books. Find out if you can request a new books for check out. Our local library just ordered a whole U.S. History series for me to check out! It never hurts to ask!
Happy curriculum hunting!