Sioux City does have an airport that is serviced by United. There are direct flights from Denver, CO and Chicago, IL. It would also be possible to fly into Sioux Falls, SD which is an hour north of Sioux City, or to fly into Omaha, NE, which is an 1 1/2 hours south of Sioux City. In both cases get on to I-29 and drive south or north respectively. You would need to rent a car as there isn't public transportation to get from one city to the other city.