Back in the days of Napster, I found tons of rare Welles radio stuff lurking in people's hard drives. I believed they were all public domain, but that's a fuzzy issue apparently. Look for one of the sites that replaced Napster and maybe you'll find something.
I found the Ceiling Unlimited episode "Anti-Submarine Patrol" but then deleted it because it had Edward G. Robinson instead of Orson in it - I thought it was from a different series or something - then I found out Eddie was subbing that week 'cause Orsie was ill. What a bonehead! I haven't found the episode since.