itms://phobos.apple.com/WebObjects/MZSearch.woa/wa/advancedSearch
Here's the Power Search URL in Safari.
You can stash this as a bookmark in your browser, and hit it whenever you need the power search. It will switch over to iTunes and show you the Power Search panel. But let's be honest, using a browser bookmark to do something in iTunes is SO 2014. Let's level up a little.
Doug of Doug's AppleScripts for iTunes fame has a handy little AppleScript you can use for just this purpose. Doug's page walks through how to create and save the script; there are no special tools required. Using that script you can fire up the Power Search whenever you want to from the scripts menu in iTunes.
For the rental example, select Movies, then check the box for “Search only for moves available for rent.”
Using the power search for iTunes movies.
Power Search also searches just free apps, or only TV shows with closed captioning. There are far more options available in the Power Search, so if you really need to get specific then Power Search is for you.
Results of a rental-only search.
With all the content available in the iTunes store, it can be overwhelming to wade through all of it. Now the search can be actually useful thanks to the magic of the Power Search panel.