Hello, Tom--
Try the OTRCAT website; they sell mp3s of radio shows on compact disc. The quality varies from excellent to barely decipherable, with most shows being in fairly good shape. I searched their website for "An American in England" and got the following results.
http://www.otrcat.com/american-england-p-1044.html
http://www.otrcat.com/advanced_search_result.php?keywords=An+American+in+England&osCsid=g1tuivtpjpqsknjriojejj1n32&x=12&y=9
As you may know, Norman Corwin is still healthy and active at 97 and still teaches in Los Angeles. He's also active with Peggy Webber's California Artists Radio Theater. (Leonard Maltin has some recent pictures of him in the "Leonard's Journal" section of his website: http://www.leonardmaltin.com/
All the best-- Randy Skretvedt