Blizz added these two NPCs back in 2.4 but in case you haven’t noticed by now you can find Specialty Ammo Vendors in Shattrath City. Marksman Bova is located in the Aldor Rise at 37,27 while Archer Delvinar is in the Scryer Rise – 41,63.
Now you don’t have to port all over to buy your ammo since they sell ammo from both Hellfire Factions, Cenarion Expedition, Violet Eye and The Scale of Sands. Though you do have to have the correct reputation to buy them.
Taken from Wowhead: the discounts from this guy appear to be based on your reputation with the Scryers/Aldor, not the specific faction that sells the ammo. For example, my hunter is Exalted with the Violet Eye, but a stack of Mysterious Arrows sells for 95s (it should be 80s with the 20% discount).
Basically, if you slacked on repping up with Scryers/Aldor, it might be cheaper to pick up your Mysterious Arrows or Timeless Shells at the instance.