Seriously. HA/VRBO is not the only place owners advertise and when they do advertise elsewhere the verbiage they use is often exactly the same. Use that to find owners' web sites.
I did research into a place at Park City a few years back. My conclusion is that I couldn't even cover my mortgage with the rental rates I'd get for the short ski season (Dec 20 - April 1 is the prime ski period). Summer (mid-May to maybe a week or two after Labor Day) gets little traction in the mountains except maybe in Aspen where they host a lot of big-shot conferences and concerts during the summer. Mud seasons are totally dead. Take a look at the rates posted on a typical calendar in the area you're looking to see how seasonality affects rates and whether you can make the money you want if you're only half-booked, because that seems to be the norm.
It's possibly worth it if you are nearby and will want to use the place for your own needs when it's not booked (e.g., mid-week during January).
Along with looking for websites, you can also type the city, state or region plus "vacation rentals" into the facebook search bar. A good start would be searching "Rocky Mountain Vacation Rentals." Many owners have a FB page for their VR, there are also FB pages for a lot of specific areas. There are thousands of VR owners on FB forums who may be able to help you.