https://smile.amazon.com/s?rh=n%3A7141123011%2Cp_4%3AWizarding+World&ref=bl_sl_s_ap_web_7141123011

That is Amazon's Wizarding World store selection. They have a few options for backpacks in your price range.

When I went last year, there were lockers close to the ride entrance, that I think asked for a biometric scan of your finger to verify your identify, which then gave you a locker and when you've done on the ride, you go back to the same machine, scan your finger and it opens your locker, though be careful, once its locked, it won't re-open unless you get assistance! Hope that helps!

I think most of them used leather or canvas shoulder bags. Like in the books where hermiones bag rips open, she has it slung over her shoulder like a side bag instead of a traditional back pack.