Lowes generally have them but the employees won't have a clue what you're talking about.
Best one for price/quality is the ILBE GEN III military pack. $100-150 at a military surplus store. Super duable, about 80-90 liters.
That might be a little expensive and big for you though. Really man for now anything with a hipbelt and frame is gonna be so much better than a duffle.