I have a couple of students in my grade that are really great spellers and are keen to partake in a spelling bee to test out their ability. I have found a few online sites for them to try to sharpen their skills that you might like to try with your own students.
Spelling Bee the Game - Based on the movie and is a great interactive challenge for the kids. This is for your freakishly good spellers only.
Anyhow Good luck and let me know if you come across any others.