You will have to specifiy if it is a handgun, rifle or shotgun.
A person can buy a used Beretta 84 gun frame through websites like Gunbroker or at local gun shops. The frame is the main component of the gun where other parts are attached.
You will have to be more specific. Long gun or hand gun?
The frame.
It is the frame of the gun. With present day guns, the part with the serial number on it.
The receiver is the frame of the gun. When a gun is marked with a serial number, it is stamped on the receiver. It will be a different part depending on the type of gun.
The serial number is on the frame.
On the frame and or barrel
Gun shop, gun show, want ad
Normally on the frame/barrel/receiver
It is in different places depending on the gun, but typically, it will be somewhere on the barrel or on the lower frame of the gun.
With a break open shotgun, the part of the gun frame that is hidden by the closed barrels.
Depends on the parts, and from what country. The frame (receiver) of a gun is subject to import laws.