I see them come up for auction on eBay, but there is either some issue with them that makes wearing them undesirable or the price is outrageous or they’re fine and the price is reasonable, but the size isn’t good, for me. You’re right, they aren’t plentiful. It seems they’re about as available as the M-1943 jacket liner is now. A decade or so ago, you could find them in good condition, reasonably priced and in numerous sizes. Now, you can find then, but in very few sizes and they often have condition issues that aren’t easily fixed. On top of that, these damaged liners are often priced at a pretty expensively. Though, I’d say condition is the main issue...ripped shells and/or lots of missing alpaca. Seems many of the same issues are present with the existing B-11’s they are put up for auction.
Maybe it’s just the occupational hazard of collecting 70 plus year old jackets and coats, besides straight field jackets, which are just cotton, built like tanks and have, mostly, held up very well.