Can you possibly post a photo of the offending malfunction? We need to see if it’s a zipper fastener or slide or just the material, if it’s the material you may have a major issue, good luck
Have you contacted the backpack manufacturer to ask if the broken zipper is covered by their warranty?
Even if not, they may offer a repair service.
Check with local outfits that do marine upholstery, boat covers, etc.
I would check with the manufacturer first, though. It may be under warranty, and even if it isn't they may be willing to sell you a matching zipper.