I read that the ones from Pinball Life were pretty good. I play mine too much to cover them up unless it is with junk from other stuff I am fixing, LOL.
I've seen these on eBay and thought about buying some, but then my machines are side by side with no wiggle room, so I figured it would be too much work taking them on and off.
I just fold up old packing blankets (like the ones UHaul always tries to sell you) and set them on the top glass for each of my pins. Helps keep dust off and protects the glass.
I've been tempted to pick up some of these as well, at least for the head, to try and keep the dust from collecting on top of them (also in the garage). Wonder if you can buy just the head cover - the pic's on that first site seem to indicate it's two parts..
I've found the covers too much of a pain to put on and take off. To the point of not playing the pin when I wanted because of the hassle. I just throw blankets as mentioned previously now and have the covers in the closet