I homeschooled for 19 years, and there are a number of online swap boards for school books. My favorite was at Vegsource: http://vegsource.com/homeschool/
You can check out the ads and contact people that have books you're interested in. Sometimes you can dicker on the price. Going rate for a good-condition used book is half the retail price.
Also check e-Bay, but a lot of homeschoolers quit using e-Bay because they made a rule that you can't sell teacher's editions. And in some of the higher level math and science, you've just got to have the teacher's edition.
"If destruction be our lot, we must ourselves be its author and finisher. As a nation of freemen, we must live through all time or die by suicide." ~ A. Lincoln