Order an Adventurebilt, wait a year for it to arrive, rip out the liner, pour a whole bottle of white-out on their logo, let it dry, put it in your hat
Seriously though... that's a good question. Come to think of it, I have some hats that could use a liner, but I wouldn't want to go with a hatter supply company, as I fear they might have a bulk order requirement...
dj, do you need a liner, or is it just for posterity? If you've got it on your head for costuming, no one's going to see the liner anyway. Just a thought.
I don't have any plain liners at all. Except one left that was used in the film hats. And of course, I can't sell that one! LOL. You might try Art Fawcett. He probably will sell you a plain white one. Fedora