Go to any vendor where you can find any type of ammunition. Choose to purchase all of the same type of ammo, but do not confirm the purchase. After it appears on your side of the transaction screen, choose the ammo on your side of the screen and give one round of ammo back to the vendor. Then, choose to give all the ammo back to the vendor. If done correctly, you will have one round of ammo remaining on your side of the transaction screen. You can select that round of ammunition repeatedly to get all of the vendor's caps.