You can try eBay or Gunbroker.
Not ebay. At least not currently.
From
https://www.ebay.com/help/policies/prohibited-restricted-items/firearms-accessories-policy?id=4277
(Emphasis added to find the section on brass)
What is the policy?
The following are not allowed:
Firearms of any type
Parts and accessories for assault weapons, including blueprints and instructions
Any part or accessory in violation of local law
Firearm receivers, even if incomplete
Ammunition for firearms,
including bullets, cartridges, casings, shells, and hulls
The following firearm accessories are not allowed:
Frames, kits used to create a firearm, polymer 80 frame kits, flash suppressors, silencers, sears, high capacity magazines that accept more than 10 rounds (even if they have been modified to accept fewer), magazine extenders, bump fire stocks, grenade launchers, and pistol conversion kits
The following accessories for semiautomatic pistols are not allowed:
Barrel shrouds, forward pistol grips, and threaded barrels