Check your Marshall's for backpack clearance. A number of character packs -- plus name brand bags like Jansport etc.
Prices range from $3 to $10.
Marshalls - www.marshallsonline.com/
Remember Jansports have a lifetime guarantee.
If it breaks and you need it fix they will repair it or replace it.
Also there are a number of Disney packs -- great for a Disney trip.