Here are links to the most widely used PPO networks in Colorado. You can check to see if your physicians are listed. It is still recommended though, that you call your physician's office directly to confirm that listings are up to date.
http://www.humana.com Humana Home Page - Find a Doctor in Center of Page
http://www.provider.uhc.com United Health Care Network
https://www.anthem.com/health-insurance/home/overview Anthem Blue Cross Network, click "Find a Doctor" in right hand frame.
http://www.mygreatwest.com Great West Network
http://www.phcs.com Private Health Care Systems
http://www.sloanslake.com Sloan's Lake Network
http://www.covercolorado.org Cover Colorado - State Funded Safety Net for Otherwise Uninsurable