I hope it is...

The inventory changes every time, they might have one, they might not. However, if you come across a Vigilant of Stendarr, they usually wear one.

The best places to buy the Amulets are Radiant Raiment in Solitude and the dark-elf woman in the Windhelm marketplace.

Their stock is randomised though so they don't always have them and there are nine different amulets but at most they only have two sorts at a time.

Why buy it?
I've found all the amulets more than once on people I've killed in dungeons.

Playing as a Stormcloak,
And yeah, Vigilants typically carry one as well as their requisite Cure Disease potion.
Given their predilection for getting killed by everything they fight, it's only a matter of time before you find one.

You'd think they would have learned that rookie mage+robes+cave bear=dead mage, but they just keep cranking them out and sending them to their doom. Gotta wonder about their recruitment methods...

You can also find them as random dungeon loot, like any other amulet, although you can skew the odds in your favor a bit by raiding Conjurer and/or Necromancer lairs as the chests therein have a higher chance to contain enchanted jewelry.

I have several Amulets of Stendarr in the Jewlery drawer in my house. Found all of 'em in dungeons, caves or random places. Never bought one.

Your best bet in buying one would definitely be at Radiant Raiment. Keep checking there and all the other places that sell jewlery and amulets every day or when they restock their inventory.