If it's hard to find a webcam with an auto focus, why not find a digital camera with webcam capabilities. Granted, it might be pricier than what you intended.
Most webcams use CMOS cameras with fixed focal length. I have this little colour one, and the picture is always in focus no matter how close or how far you are from it.
Really the only need for auto focus is if the camera has an optical zoom function.
I think if you even managed to find a webcam with auto focus, it would probably cost about as much as a cheaper digicam... Plus autofocus isn't the best for rather fixed subjects as it can sometimes mis-focus.