Jim-
We have several sources for barrel liners. Some come from barrel makers and are basically button-rifled barrel blanks that are 1/2"-3/4" in diameter, others are made specifically as liners on a hammer forging machine from suppliers that just make liners. These are not like the aircaraft tubing 22 liners you get from Brownells, they are truly rifle barrels, just very skinny. We drill out the original barrels, ream them, fit the liners so no joining lines are visible, crown/chamber, then reblue. For a collectible or a rifle that screw-in barrels are not available for, it is a good option.
As you said, most people who want a barrel lined have a collectible or heirloom rifle they want to keep original in appearance and restore to shooting condition, others simply want the rifle returned to service. For many years, barrels were cheap, cheaper than or close to the price of paying to have the barrel drilled and reamed. liner installed, then chambered and crowned, but barrel prices have headed north over the past few years with the increase in metal prices. We used to do just a few a month, but now we do several a week. I relined two just today, both collectibles, but I did one last week that was a relatively common rifle in 30-06. Just happened that the price for a relining was the price we got back for just a barrel blank, almost to the penny. And for folks that want a less expensive option to rebarreling, we also rebore and re-rifle barrels, mostly for BP cartridges, and that is another thing few people do any more.
Some folks don't bat an eye at dropping $250 to $450 for a barrel blank, $200-$300 for installation, and then shoot factory ammo in the rifle, others don't see the value in that. We serve both groups.
And sadsack, I wasn't trying to be harsh, so I am sorry if it came across that way, but there are a lot of gunsmiths nowadays that only know how to replace parts, they know very little about actual gun smithing and making guns and parts, and that is what we do. I hope you won't hold it against me.