Duping is a glitch - taking advantage of a glitch is a form of cheating no matter how you look at it. Personaly I don't care wether or not you cheat, but don't tell others to do it just cause it may be an easier way. If they want to dupe that is their choice. TC asked where he could BUY arrows, and not how to dupe them.
To answer the TC - As previously stated, the Archers Paradox in Bravil is your best bet. You could also go around killing bandits and looting their arrows.