If you do buy a replacement vent liner from someone other than the company that made the gun, be prepared to modify its length.
Almost all threaded vent liners are intentionally made longer than they should be for any given gun.
If a new, unmodified vent liner is screwed into the side of the barrel it will protrude into the inside of the bore.
Not only will this keep a cleaning rod from reaching the face of the breech but it will cause mis-fires.
Once you get your new vent liner, remove the original and compare the lengths.
File the new liner so its length is the same as the original.
Holding a threaded liner while your filing can be a problem unless you use this trick:
Buy a hexagon nut with a thread size that will fit the liner.
Screw the new liner into the nut with the inner end protruding from the nut face.
Adjust the length of protrusion so it matches the amount of material that needs to be removed.
Clamp the nut in a vise or, use some self locking pliers like Vise Grips ® to firmly clamp on the hex of the nut.
This will cause the nut to collapse slightly and firmly grip the threads on the liner.
Use a metal cutting flat file and remove the protruding threads down to the face of the nut.
Remove the nut from the holding device, unscrew the liner and your done.
You might want to lightly chamfer the filed threads to remove the sharp corner that was left from the process.
Have fun.