are you looking for any group exercise instructor certification or do you have a specific organization in mind?
If you are looking for a specific organization, then the best place is to go to their web site. If proximity to Lincoln, NE, is your most important criteria, then I would suggest that you contact aerobics coordinators of clubs in your area. They are probably on mailing lists for organizations. AFAA tends to have many workshops all over, and you can check their web site http://www.afaa.com/ where you can search by ZIP code.