Several companies offer the liners including Savage. You can even make your own with a little skill. They are roughly $4-5 each. If you get some from Savage, get a extra breach plug at the same time.

Smokeless loads are much more sensitive to the flash channel getting full of carbon so many people just swap out plugs and then clean out the one they just removed. This also gives you a chance to inspect the size of the flash hole in the liner. Once the vent liner is eroded to about .035+ most guys pitch them.

BTW the BEST thing about a Savage IMO. You can convert them to a deadly accurate 45cal for about $460 or less. And that is for a extremely good PacNor barrel. Swapping the barrels is easy with the tool and you can even head space it yourself pretty easily. Most of these good 45cal conversions are shooting a sabotted 200gr 40cal over 2800fps very accurately.