I have been in the process of figuring this out too. Initially I was planning on taking a regular underdash speaker and trim off the excess metal, then I saw that custom auto sound has a speaker that is the same shape as the hanging style. What I was planning on doing was taking the speaker grille to a place that sells speakers and seeing how it lines up with the speakers they have.
http://www.custom-autosound.com/prodList.php?FamID=1