These folks maintain an exhaustively complete catalog of bluegrass, old-time, early country, and folk music recordings. The Britsish Bluegrass Music Association has a nice web site - you can even read their newsletter on line. The Fretted Instrument Guild of America has a site - an important organization for acoustic music luthiers and repair techs. Mugwumps Online is the Web site for the venerable publication for serious musical instrument collectors. Bluegrass Connection is a must see - tons of artist information including lots of sound files from recent recordings.
Homespun tapes has their catalog on line.
These are invaluable learning aids for players at any level. EMusic - Another online CD source. They did not have as much bluegrass, but here it is all the same. D'Addario Strings are well known to bluegrass pickers. The House Of Fine Instruments has several new pages of instruments and accessories and parts, as well as a links page.