I am a boy scout leader looking for ideas for a pack Halloween party. The boys are 1st grade through 5th grade. The best time to buy backpacks and lunch boxes is in September, after school has started. I have found fantastic prices at Walmart in mid-September. If my kid wears out or rips the backpack, the company will repair or replace it for free.
Several companies have backpacks with lifetime guarantees.
The ones I bought are made by Outdoor Products. I wrote them to see if I needed to send anything in to register for the guarantee. They said no, just send it in and they'll take care of it free of charge. Look around for special offers and giveaways. Our local minor league baseball team gave away free backpacks for anyone who attended one of their games.