If you have the Misdirection perk, you can pickpocket it from him. Falas does not sell it, nor can you obtain it by killing him, as he is marked as essential.
This bow is relatively difficult to obtain. Its enchantment bestows the Blessing of the Stag Prince while the bow is equipped, which increases your health and stamina by 5 points for every 20 animals killed by the bow, maxing out at 25 points after killing 80 animals. It is identical to a normal glass bow, but it deals one more damage.
The Glass Bow of the Stag Prince is a glass bow wielded by Falas Selvayn, who can only be found at the Ramshackle Trading Post between the hours of 12am and 5am.