Head on over to the 2 archers firing arrows at a circle in the middle. Go to the one on the left as he fires quicker and plant something on him say a pie or ale and wait till around 8PM, that's when the archers go to bed.
Whenever the go to bed go into the castle called the Castle Dour.
When you are in the castle take the first right and go downstairs. This is where the archer will be sleeping. When you reach the bottom of the stairs take a left and select the option to pickpocket every guard UNTIL you find the one you planted an item on. Whenever you find him you want to take his steel arrows and then go to your weapons in the pickpocket menu and you are going to give him one of the arrow's you want increased e.g. Deadric arrow's. Then just wait until they wake up at 8AM and go outside and you will see the guard/s firing your arrows, all you simply have to do is take them and that's you! You will now have the desired amount of arrows you want. Remember he will shoot all day and no you can't wait until 8PM then pick them.
Also if you are apart of the thieves guild in Riften
This video is to show you where they shoot their arrows in the thieves guild. Just simply do the same steps as above to get a certain amount of arrows of your choice!
I hope this tutorial has helped you and their are alot more on the way! Thank you.