The Train Department has 1/16 & 1/8 inch 12 x 12 ceramic insulation sheet for $7.00. I have used the 1/8 " to insulation a Stuart 501 boiler. I used the 1/6 " on my locomotives. Works good.I assume you are looking for a sheet material.
There are modern versions of thermal barrier sheet available, it’s just finding a suitable supplier.
Try searching for ceramic paper, or insulation, and look for something like this.
Amazon.com: HMFC 12"*24" Ceramic Fiber Fireproof Paper : Tools & Home Improvement
There are all kinds of thicknesses available.
Kaowool is a much thicker material and good for the end of smokeboxes, etc..
David Leech, Delta, Canada