Pretty sure they're custom-made. Harry not being able to get one was kind of a thing in the story, which would've been weird if he could just have taken one off a dead Warden.
Anyway, this is from the wiki. Don't remember who wrote it up originally.
WARDEN SWORD [-1]
Description: The Wardens of the White Council are famed for their magical, super-sharp, spell-cutting silver swords. This is one of those swords. It was created by Luccio, the leader of the Wardens, specifically for a single Warden.
Musts: You must be a Warden of the White Council, with an Aspect reflecting that.
Note: Warden Swords are canonically Enchanted Items. This is an alternate interpretation.
Skills Affected: Weapons.
Effects:
[-0] Specific Wielder. A Warden Sword is only magical in the hands of the Warden it was made for.
[-1] It Is What It Is. It's a supernaturally sharp sword. Weapon 5 for the rightful bearer, weapon 3 for other people.
[-0] Unbreakable. As an Item of Power, this item cannot be broken except with a magical ritual that opposes its nature.
[+2] One-Time Discount. A full-sized sword is tricky to conceal at best.
[-2] Might Over Magic. A Warden Sword can cut magical spells the way a laser cuts butter.