Toys R Us is running a nice offer again- get a free lunchbag valued up to $9.99 for free with purchase of a backpack.
They typically run this promotion for several weeks- pretty much up until school starts however it is important to note the selection tends to get more and more picked over each week.
A few years ago I wanited until right before school to get this offer and there was so little to choose from.
This is usually one of the better deals of the season for bags. A lot of people like to wait for Office Max too. They typically offer a free after rewards backpack sale at some point during summer.