Almost anywhere that sells AR-15's, sells commonly needed parts for them.
Brownells.com and MidawayUSA.com have every part for an AR-15, and there are plenty more parts sellers on the Internets.
The 2nd Amendment was, and still is intended to protect the individual citizen from Tyranny. Whether that Tyranny originates from
our own Government, common criminals, Progressive Liberalism, or foreign invaders, is irrelevant.
No luck online
I did find a Buffer retention spring locally. I took it to local hardware store and matched it, bought 10 they had for .54 each. Found out that breakdown detent spring is same as a cheap zippo style lighter under the flint. I had a junk one laying around. thought I would pass that on.