Many stores, both physical and online, stock many backpacks that feature wheels. The most prominent online store is most likely Overstock. They are well stocked with low prices.
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When you get to the right rank, you get the backpack as a reward. The backpack is permanent (and free).
buy them at cybermall
Do you mean one that goes on your shoulders and back, or when you enter the game you get certain items.
The blue backpack is in the Penguin Style catalog every once in a while.
If by "backpack" you mean "Ava's Attractor", this can be obtained by completing the Animal Magnetism quest.