You can also make your own microwave bags pretty easily. Just throw a 1/4th cup of kernels in a small brown paper bag, and fold it over. I usually drop like a tsp of oil in there, and a bit of salt, before I pop it, and shake it up a bit but it works just as well dry.
A while back though, i bought a microwave popper off Amazon though. A glass vessel that kind of looks like a coffee pot with a silicone lid. Same idea without the greasy bag, and I can just take that with me as as the popcorn bowl.