I'm going to pass off a tip I've learned through the years of buying homeschool supplies. It's pretty simple. Decide what you want around March or April, check prices at various suppliers (plus shipping), and then go to eBay for those items. If you do that, you can walk away with some really good deals! (Figure in shipping when you bid though, because getting "really good deals" can be quickly offset if you are paying higher shipping than you would if you buy brand new from one supplier. Many times, they offer free shipping on large enough orders.)
It has always amazed me to see people bidding for used materials on eBay and paying more than they would from the regular supplier for brand-new stuff. It seems like the time period is getting earlier and earlier though. Last year, April was plenty early---now more seem to be aware but it's still better than buying in August!
Of course, if you're just buying blindly, stop! Unless you want to rival the county library of course! ;-)