I have these, and I wish everyone did, but I've never seen them in a store around here.
I read somewhere recently (maybe here?) that it was important that you vent a dish in the microwave, because microwaves have moisture sensors that help determine when to end a heating cycle. Upon reading this I was eager to dig through the boxes I shipped back from my apartment in Chile and find the plastic dish covers that I bought there for use in the microwave. It's a nested set of three sizes, and around the perimeter are vents, so that even if a particular size fits snuggly on the bowl or plate, air can still circulate around the food. I presume this is because of the aforementioned moisture sensors in the microwave.
Back when I lived in Chile, I found these covers so useful (in addition to helping control a dish's moisture content, they also contain splatters), that I used them constantly. I must have bought them in the Jumbo or some other supermarket in Santiago.
Now I wonder why I've never seen these for sale in a supermarket, Bed/Bath/Beyond, Linens/Things, or any other sort of cookware supply place here in the US?
I have mine so I don't need them, but I think everyone else should have a set.