Every now and then you can still find some the old H Bar C or the old California Ranch Wear in vintage stores or even on ebay. For the past few years you could even get NOS H-Bar-C stuff on ebay.
If you want new stuff
Rockmount Ranch Wear is making some great vintage looking shirts from some of their old patterns and they are the longest production shirt design in America(they have been making the same shirts since WWII).
Papa Jack from Rockmout was the first to use snaps on western shirts back in the forties and he is still doing it. I think he just turned 103 this past March and still shows up for work every day at the factory in Denver.
I have a few old H-Bar-C shirts that are to small for me anymore if anyone wears a small let me know.