All of COS is handled by SkyWest. Mainline was taken out back in January, but some of the mainliners stayed with SkyWest.
Mesa is horrible there, along with DEN. Never fly a DH8. You will thank me later. They suck to fly, and suck to load.
Your best bet is to fly out of DEN, and work in COS. You will thank me for that as well.