It's been a lifetime since I've been there, but if you're looking for a local place to "window shop",
Gateway Electronics off Page Avenue had LOTS of electronic/electrical goodies.
Also in STL on Page Avenue up north is McMurray Music Center, but my gut feeling is they may have gone out of bidness.
As far as poly stuffing, the material you'd use to stuff blanket/comforters is also in Wallyworlds sewing department. It comes in a bag big enough to hold two or three basketballs.
Not sure about it's acoustic qualities though.

Bob