I put a small computer fan that I got from Radio Shack
Many such muffin fans are 12V DC, intended for the inside of computers where 12V is universal. Your fridge may not have any 12V supply, so you'd need to add an adapter brick. A few years ago I found a few 120V AC versions of these fans, can't remember where, probably a small electronics component store (you know, like a place that would have parts for Ham Radio, vacuum tubes, and more kinds of watch batteries than you could believe). A quick google search
shows a few as well. Good luck.