It's a really painful thing to hear a song you love and then not being able to actually find it. I've had this in the past a lot and have even spent 6 hours a day for many days, trying to find a certain song.
To date, the only one not available anywhere is a song called "Driven to this" from the 2000 film The Watcher. That one does not seem to exist anywhere and was probably only made for the movie. Which is sad, but at least it's the only one I could never find. XD
So, as sad as it is to find such a song in a western production, imagine how annoying it is to find it in some relatively unpopular (at least with non-asian audiences) series and in a foreign language. This happened to me with God's Quiz and the songs cannot be found in the osts for the series, of course.
Oh, the hours I've spent googling "korean rock", listening to the songs in the episodes and trying to find the proper romanized version for what I heard and recognized, turning them into hangul and googling them, hoping to find the titles of the songs... But it paid off. I have now found all the 3 songs that I really liked. So, without further ado, here they are. ^^