Radio Spirits has slashed the prices on their Superman sets down to $4.50-$10.  (One set had originally been $39.98.)

SUPERMAN: UP! UP! AND AWAY! is a two CD set containing the first twelve broadcasts and trading card:
http://www.radiospirits.com/detailsv2.asp?mbprodid=55009&sid=MjMwODU4Njc6NC8yNi8yMDEyIDk6MDA6NDYgQU0=&Svr=.62&l=1&source=&scode=G10CY100

SUPERMAN: THE LAST SON OF KRYPTON.  10 CDs of episodes from the late Forties, plus a booklet on the show.
http://www.radiospirits.com/detailsv2.asp?mbprodid=55001&sid=MjMwODU4Njc6NC8yNi8yMDEyIDk6MDA6NDYgQU0=&Svr=.62&l=1&source=&scode=G10CY100

THE ADVENTURES OF SUPERMAN: BATMAN'S GREAT MYSTERY.  The classic crossover adventure with the Man of Steel and the Dark Knight.  Curiously, this 3 hour/11 episode serial is only available on cassette.
http://www.radiospirits.com/detailsv2.asp?mbprodid=54076&sid=MjMwODU4Njc6NC8yNi8yMDEyIDk6MDA6NDYgQU0=&Svr=.62&l=1&source=&scode=G10CY100