Gillian Welch is exactly what you're looking for. Any album will do.

There's others out there still playing real folk and more traditional country music. Not necessarily with just a lone guitar though, sometimes with a full band.

Other good ones are:

Greg Brown
Laura Cantrell ( http://www.lauracantrell.com )
Richard Buckner
Iris Dement

also check out Loretta Lynn's new album "Van Lear Rose".