Are you talking the extra long size one buy's for college length dorm beds? Since you are from the Saint Paul area, most college towns stock the extra length sheets for those dorm beds. All bed, bath, home and target stores carry them usually.
Or are you talking a regular twin bed and you're disappointed about not finding cute little kid sets that you would normally put on a crib size mattress in the proper length for a standard twin? You don't need extra long twin sheets for a standard twin bed. Any size twin will work. Then decorate with your little kid theme, as in pillows, quilts, etc.