I shop and stock up when there are sales. Usually Wal-Mart has the best prices.
This past September, we doubled in kids- had 4 kids under roof and it put a hurting on my budget.
I watch the sales & keep a plastic tub of supplies to draw from through out the year. It's crazy.
If you have a store called Big Lots in your area, I suggest you check them out. Not only are they good for school supplies but gardening, toys, household goods as well.