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i'm gonna get a lot of hate for this but...
Steal one from a store; no troll.
In the US, the stores that sell these backpacks (dicks, gander mtn, bass pro shops, etc) have terrible security and often no-chase policies. You can just take the pack you want, put it on and walk out. Most of the time, the little alarm sensors don't even work. Just don't park nearby or use another form of transport and don't be dumb about it.
If you really want to /out/ but can't afford it, cheat a multi-billion dollar corporation out of the $200 or less for it.