iTunes Plus songs are songs that contain no DRM so they can be copied repeatedly. But be forewarned, even though there is no copy protection, the songs most likely contain watermarks so that their origin can be traced.

The file extension for iTunes plus songs is .m4a. This is extension is *not* exclusive to iTunes plus songs, other music out there may use the .m4a extension. Nonetheless, you can start using Google to search for iTunes plus songs. Here's the search format you'll want to use:

-inurl:htm -inurl:html intitle:"index of" "Last modified" m4a Red Hot Chili Peppers

In this example, results will be returned for "Red Hot Chili Peppers". Replace this section of the search with whatever song, band, or singer that you are looking for. Right now there are not a lot of results out there since the iTunes Plus format is relatively new. But I suspect that you'll find an increasing number of results will become available as more people begin to use this format.