Tom Russell. His early work with the Tom Russell Band is my favorite era but he continues to put out excellent cds.
Jerry Jeff Walker. I'd recommend only his early work, up through "Ridin' High".
I don't know if either sound like Jamey Johnson, though, because I've never heard him.
Some others worth looking into:
Joe Ely
early Steve Earle
Billy Joe Shaver, especially the band Shaver with his (now deceased) son Eddie on guitar
Dwight Yoakam
Ray Wylie Hubbard
and anyone with an interest in country music should get familiar with Emmylou Harris. I'm sure she doesn't sound like Jamey but man is she good.