Matthew,  You can get suitable felt from a piano parts supplier.
Two grades are available, cushion and check, both in a variety of
thicknesses from around 2 mm to 6 mm.

Over here I use Heckschers in London but there must be similar
suppliers in USA.  For small quantities you could try John Farmer
http://www.musicanic.com/index.htm but again there must be suppliers
of smaller quantities in USA.

For impact cushioning I would use cushion felt.  Check felt is the
much harder stuff used, as the name suggests, for the back checks
on piano actions.

Best wishes,
Nicholas Simons, GB