Do an internet search for "royalty free music" or "buy-out music" and you will find plenty of content you can purchase and legally use. Then it's a simple matter of either playing that content on a server with a soundcard near your PBX, or burn it to a CD and use a cheap CD player on repeat mode, or as suggested purchase a digital looper box.
If you want to mix in corporate onhold messages, there is a GREAT shareware program called MultiQuence (
www.goldwave.com) that makes multitrack editing a breeze.
Peter