I would also recommend placing an ad in the "Parts Wanted". Also, call "Delks Army-Navy Surplus" to see what they have.
I don't have a specific mfg. recommendation/source on sound deadening replacement material, however there should be lots of options used in automobiles/trucks & general audio installations for sound deadening. Just need to make sure it meets flammability/temperature requirements and has the sound attenuation factor you desire.
Hopefully, there are other SS members that can chime in with a specific recommendation that they know works.