Does anyone know where I can find a list of short stories by Ray Bradbury that have been published in Playboy?

I don't know where such a list would exist, but I know that you could contact Playboy magazine directly and I'll bet they could provide such a list. Anything that makes them look like a contributor to American literature (providing more excuses for people to buy their magazines, while claiming they are buying them for the stories) would be of interest to them. (It would be a public relations victory to them.) I bet you could ask them to do a short article collecting all the stories and giving short summaries of them with a listing of what was published and when. Maybe I'll try this myself.

The other possibility, of course, would be to find some "collector" and sit down for several days and go through them one at a time. (There could be worse ways to spend a day, I suppose!)