How to find websites marketing private house listings for sale?

Apologies if this isn’t the right sub. I’m seeing if you SEO folks might know how to reverse engineer for someone that is *searching* for specific types of websites, in this case home listings. For example, sometimes real estate agents (I am one) will have websites made specifically for their private listings that are not on the major websites (Zillow, Redfin, etc.) Sometimes the website is the property address (like 123mainst . Com) I am wondering if there could be a way to find these websites without knowing the property address. Any ideas, or other subs that might be better for this question? TYIA