How do I find where the file for a particular mp3 that I can see in iTunes is?
Select the song in iTunes and type &#x21E7;+&#x2318;+R to trigger Show in Finder.

Alternatively, you can also right-click the item in iTunes and choose this option from the contextual menu.

From there, you can navigate up the folder hierarchy by holding the &#x2318;-key and clicking on the folder in the window title bar. You might also want to enable Show Path bar in finder.