One more for the bargain hunter:
Find backpacks at garage sales and thrift shops, such as Jansport, with 'lifetime warranties.' If they are torn or missing parts, Jansport (and probably others) will repair or replace the item. (PS to those with real money: Buy new things from Jansport. They step up to their warranty wonderfully, and it's the best damn marketing device out there. Others are smart like that too. Notice how I just did advertising for them they didn't have to pay for.)
The Weasel