You can go to the regular Kindle eBooks section of the site and browse to whatever genre you want and then just check the box on the left side of the page that says "Kindle Unlimited Eligible".
This link for example will show you all 641,273 eligible books in the non-fiction category which can then be narrowed down by selecting from various categories on the left side of the page...
https://www.amazon.com/s/ref=lp_1573...nid=9045886011