What's your preferred convention? Comics, anime, fandom specific, or other? I'm not sure of any offhand in South Dakota, but I know that Chicago has a handful of conventions of different kinds and that there are some in different parts of Ohio. And I believe that Wizard has a comic con in Minneapolis.
But there are also so many smaller cons that happen all over the country which are more focused on particular things that it would probably behoove you to hit Google to be able to pinpoint some closer to your area and interests.
Heck, there was an anime convention in my city a few months back that I'd never heard of and didn't even know about until I was driving by the convention center and saw a bunch of people in costume. And I've lived in this area my entire life, so it'll probably take some digging and searching to really find everything that's out there for you to go to.