In my youth I inherited a Boy Scout Haversack style backpack…
I had that thing from 5th grade all the way through undergraduate school and it was the most durable backpack I’ve ever seen.
My friends in collage used to buy new packs about once a semester, but my Haversack saw me through 4 years of college when it was already an est. 10 years old! The old one finally gave out and had to be retired.
I LOVE this backpack - please tell me I can get a new one!
Army Surplus stores are your best bet. 15 years isn’t that great for a backback, though.
Checking out the Boy Scout’s on line catalog I don’t see what I recall as the traditional scout backpack (all canvas, roughly rectangular, no zippers, and a flap over the opening)
You might try buying a backpack at a mountaineering store. My kids used to go though a new backpack every year. At about $35/ per I was not happy.
Then I got the bright idea to go to an outdoor store (A16 here in LA or REI) in both my kids cases, that made for the last backback I had to buy them. My daughter’s is about 7 years old, and my son’s is about 15 years old. Well worth the extra $20 up front.
I got a cool little heavy canvas backpack from an Army Surplus store. This was about twelve years ago, but I paid $15 CDN and used it to haul schoolbooks for ages. It never ripped or anything.
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My best bookbag ever was the bag from a '50s era satchel charge that I got when I was a jarhead. Don’t think I’d carry that around nowadays…
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As always. 
The only thing is though, I’m not sure how many you’d actually find on there that would be new. The good thing is that you can have them to alert you via email for anything you’re looking for. And, you could get even more search results with just “haversack” (except without the quotation marks), but I didn’t want you to have to look at a whole bunch that wouldn’t initially suit your purposes.
Oh, and don’t forget an * at the end. Makes it easier for dealing with capitals and the like. Good luck!