This is your one-stop shop to practically all TV theme songs (and they must have at least a thousand recordings): http://www.televisiontunes.com/ Not only that, almost all of them are original recordings, just the way you remember them. However, that is both a curse as well as a blessing. Most have been "ripped" right off the TV, practically. The original recordings are not stereo, not digitally remastered or enhanced in any discernible way, and sometimes are just plain bad recordings that sound like they were made by somebody holding a cassette player microphone up to a 10" portable TV speaker and taping it. That said, you can not only listen to everything there in its entirety for free, you can also download for free.
And now a word about 12 O'Clock High. While the TV Tunes album mentioned here had its failings, it actually has a very good reproduction of the closing version of this theme done to the original arrangement which I can recommend wholeheartedly, and which also has the advantage of being a stereo studio recording. Back when I first got the album myself, and having not seen the TV show in a number of years by then, I actually mistook their version for the original; it is that faithful.