Great question! I always loved the score and rare songs in the Universal horror films of the '30s & '40s. None of the music I really loved showed up on the few soundtracks I came across. That is until two years ago when some net music shop released a compilation of rarely released music from such films. Guess that was a 35+ year wait (from when I was a horror film addict kid).
Film music, for me, is always the hardest to find when it doesn't show up on the soundtrack (if there is a soundtrack). I'm still looking for songs from the original 1973 "Walking Tall" movie. Not the Johnny Mathis title song but the soul songs featured mid-way through, never mentioned in the credits.
During my first year online, I found an Argent LP now available on cd after a 20 some year search. My record want list shrank considerably thanks to the net (and so did the contents of my wallet!).
Currently I'm trying to track down a very rare soundtrack to the cult film, "Wild Zero", released sometime last year. I think it's on Sony Japan though not positive. If anyone happens to see it somewhere, please send me a PM.
