I'm in complete awe of you that you designed and made a backpack. I almost always carry one, whether mini, medium, or carry-on large.
All we have here is a Jo-Ann's and they have very limited fabrics, even more so when it comes to utility. I haven't branched out to buying online because I don't like buying if I can't feel the texture or see how the fabric drapes first.
And I don't need anything particularly special -- I start a lot of projects "just because I want to see if I can do it."
But by the time I bought all the supplies, binding, zippers, and specialty materials... I'm thinking it's not worth it because I could just get one that would do the job for maybe $40, which I'm guessing would be a lot less than tracking everything down (and paying shipping) for everything separately.
If you're ever able to post pictures of that backpack, I would LOVE to see it!