Music used for spot and promo production ...

Hate to use the phrase, but "in the beginning" we old radio folk mostly used music from movie soundtrack and instrumental albums for production.  Back then, LP's were recorded with original music and movie soundtracks were developed with background music that enhanced the plot. 

That included original musical compositions. Such tracks were dramatic, dynamic, comedic or just come-along-with.  We grabbed tracks that worked with whatever we were selling for use in whatever the spot was we sold.

By the 1970's, along with soundtrack LP tunes, we started to use custom recorded music, tracks timed and paced for specific purposes, with special effects for spots and promos. Most of these came from Tanner and TM production libraries.

So where did these clips come from?

During my time at Tanner, they came from Poplar Tunes, the greatest record store on Earth (back then), or at least in Memphis.  Once a month, I'd visit Pop Tunes and go through their collection of soundtracks.  If something looked or sounded cool, I bought the album and took it back to the home studio.

Then I tracked through the LP's for hooks and recorded them onto tape.  Hooks were sent to the music department (our guy was Dee Barton, a Jazz pianist who once worked with Clint Eastwood) for reassembly into similar but close arrangements.

Then someone worked them into :60 and :30 and :15 and :10 tracks.

All that's all over now, with the fees charges set with production music.