I'll add that if there's a cake that you could just use real popcorn on, do that, it's a lot faster! But if you want to be fancy and impress people, here's a video showing how I figured out a way to make realistic gumpaste popcorn without cutters.
This is the kind of thing I would sell online, but it's a little time-consuming, so it ain't cheap...Honestly, I'd use real popcorn on it, probably kettle corn (which is the best kind of popcorn if we're telling the truth). Try it out and let me know what you think of this way of making it, or if you can figure out another way to make it better!
Kara Buntin owns A Cake To Remember LLC in Richmond VA, and cake supplies online at www.acaketoremember.com and www.acaketoremember.etsy.com