Internet Archive - An excellent site for finding a wide range of OTR programmes in mp3 format, especially those provided by the Old Time Radio Researchers Group, including:
A complete set of CBS Radio Workshop episodes here.
A complete set of Dimension X episodes here.
A complete set of available Suspense episodes here.
A complete set of X Minus One episodes here.
My Old Radio - Another good source of OTR programmes as mp3s.
Old Time Radio Catalog - OTR shows on MP3 CD - offering an extensive collection of comedy, music, mystery, and more.
Radio Days - A good website about all things OTR.