The backpack could be mass-produced and sold under numerous brand names. But if the Ozark Trail logo is on there, it’s exclusively sold at Wal-Mart which narrows down where the perp attained it. Wal-Mart in-store, online, possibly Amazon, or second hand.
I research wardrobe for movies & tv as part of my job. I haven’t researched these photos yet. They creep me out, plus there’s lots of researchers better at it than me. I’m sure they’ve figured it out. It may take awhile to track down the buyers of the backpacks, but it’s a shot.