I've felt pretty doggone fortunate for the past few years in the curriculum department. Our Time4Learning program included everything we needed for math and language arts, and we simply had to supplement (usually with free stuff online) for other materials. But once my youngest had aged out of T4L (except for Algebra this year) it was back to the curriculum grindstone. Grrr. We're very interest-led, so a lot of our materials come from whatever we are studying at the time. Otherwise, we check Homeschool Buyers Co-op for deals, try to buy local books, or fall back on the Amazon.com bandwagon.
I know homeschoolers buy materials all over the place...and the biggest response, if I had included it, would more than likely be: "Wherever they are the cheapest!!" But if you had to choose one MAIN source where you buy the majority of your materials from, what would it be?