Hi,  Columbia Organ Leather Company sells felt in various
thicknesses.  Their web site is  http://www.columbiaorgan.com/ 
It's best to call them and get the details.  They are great folks
to deal with, both personally and business-wise.

Another good source of felt is Organ Supply Industries in Erie,
Pennsylvania.  Their web site is  http://www.organsupply.com/ 
They have felt in many sizes and colors.

Regards
Craig Smith