Follow the link to an eleven minute video of Cleese describing his creative process and how interruptions damage creativity. The video is interesting in it's own right, but also Cleese discusses where he gets his ideas from.
"I get them Ken Levinshaw from Swindon who mails them to me every Monday."
There you have it.
Added much later (April 14, 2012). At Boingboing is a video of Cleese giving advice on how to be creative. I am a huge fan of Cleese but feel the videos may fit together quite well. This video is 35 minutes long so I presume it covers more and different material.