Filson made the finest flannel shirt that you could buy ("Might as well have the best" is the company slogan). -Do they still or have they sold out and gone to sh*t? Great American brands get taken over and trashed so quickly now-a-days that I can't keep up.
The good news is there is a vintage brand that is pretty unknown I believe so you should be able to pick them up at vintage stores or ebay cheaply. The brand is Frost Proof. Made in the US (of course), these were made during the middle of the last century and were often available at farm supply stores and the like where they were sold to guys who needed a tough, warm shirt.