The best time of year to buy these supplies, of course, is during the back-to-school sales. However, I also find excellent deals on binders and sheet protectors by watching Amazon.com and Ebay.
Typically your average weight sheet protectors will cost 10 cents/piece. So anything better than this price, you've found a deal! I usually buy these in bulk when I find a good deal.
I will do the same with binders. I buy minimally at full price and watch for the better bulk deals online. An average 1.5" D-ring (or EZD, slant ring) binder will cost $4-5. Anything less than this is a deal.
D-ring binders are more expensive. I prefer this style of binder because they allow the kids' notebooking pages to lie flat when opened ... less wear and tear on their pages.
Have more suggestions? Please note in the comments below!