It might not be appreciated by some (linking to the Reaper website), but while looking for tracks icons, I ran into these:
http://stash.reaper.fm/theme/583/SD%20JD%20Track%20Icons%201
http://stash.reaper.fm/theme/584/SD%20JD%20Track%20Icons%202
http://stash.reaper.fm/theme/587/SD%20JD%20Track%20Icons%203
http://stash.reaper.fm/theme/588/SD%20JD%20Track%20Icons%204
I haven’t tried them yet (C7 is still in the mail), so I don’t know how they will turn out…