You might find what you need at webdept@gunpartscorp.com .
Generally, you can't. It might be possible with Browning, but, you will need to call them with the serial number.
Remington
If you can find one, yes.
You find a gunsmith.
== ==
None published. Try shotgun collectors.
NUMRICH.com
Under trigger
Not enough information.
It is difficult to find even 10 gauge shotgun ammo today. I have my doubts if 8 gauge shotgun ammo is even made any more, but corrections welcomed.
You might find one at a gun shop, gun show or at e-gunparts.com