Hey there Jack!
I came across Western Textiles & Manufacturers, they have some cool looking backpacks: http://www.bagmakers.com/backpack.htm. They also have a pretty rad "making the bag" video posted here: http://vimeo.com/21861926. I would give them at shout about the custom backpacks to see what your options are!
If you are looking to do custom tote bags down the line, you can use a free application called Printful! Check it out here: https://apps.shopify.com/printful
Hope that helps!
Liz Bertorelli :: Shopify Guru