Another vote for Will The Circle Be Unbroken (Be sure it's Volume I, the double CD) as a starting point. It has some of the finest players of these genres ever assembled. Earl Scruggs, Doc Watson, Merle Travis, Mother Maybelle Carter, Roy Acuff Johnny Cash to name a few as well as some who were considered newbies at the time who have gone on to play wonderful music. Choose some of the performers that you like and explore their own stuff.

Another couple of good ones are "The Stanley Brothers, The Complete Columbia Recordings" and "Flatt & Scruggs, 20th Century Masters, The Millenium Collection" which is put out by Universal/MCA.