There are a virety of materials that can be used if an exact origional replacement cannot be found.
1. Black velvet from a fabric store.
2. 1/8 inch thick black felt from fabric or hobby/craft stores.
3. heavyweight black cotton fabric.
Use http://www.homedepot.com/p/Motsenboc...5#.Un6zo-JanJU to remove the old material without damaging the wood back and its finish unless you gouge it scraping.
An excellent product for attaching the new fabric: http://www.homedepot.com/p/DAP-Weldw...5#.Un60s-JanJU