Pick pocket the arrows from the NPC's who are shooting at targets. You have to do it before or after they are actually shooting at the targets unless you have the steal equipped perk unlocked. Then replace the ones you took with 1 arrow of the type of arrow you want to have a lot of. Then when they shoot the target you collect the arrows that hit the target and it will always be the arrow you gave them. NPC's get unlimited arrows to shoot so long as one arrow is on them they will continue to shoot that arrow.
My example was i joined the thieves guild and pick pocketed everyone in there and gave them all 1 deadra arrow. Now i go from the main room to the training room and collect all the deadra arrows they shoot at the targets to refill. I can get a stack of 100 or so arrows in about 7-10 min of running back and forth. GL to ya.