I have, on occasion, heard radio announcers say they retrieved a piece of music from the station's vault and played it on the air.
From that, I jumped to the conclusion that radio stations have available a big library of music from which they select something when the need arises.
If you were starting a new radio station, how would you go about acquiring such a music library? Do you have to go searching, one by one, for each piece of music you want - or can you buy an entire library at once? If so, where?