Here is a distributor for all things mechanical.... My goto bible for machine design... and many other applications. Anyone can purchase from them.
https://www.mcmaster.com/#felt/=1a8v2d0
Read the part at the top...
"
F26 felt is the softest wool felt we offer and is often used for packing and padding applications where the felt will be held between other materials.
"
its not cheap but a twelve by twelve square half inch thick is seven bucks.
That would make two inserts. You can also go to a shoe repair person and he may have exactly what you need as well as be able to shape it to you and your boots....
Just some thoughts....
deb