One of the constant issues with Hunter Safety is the shortage of instructors for the number of students. We are all hunters and fishermen(women) and summers and falls are busy for us as well. If you cannot find a class look into other options, dependent on the age of your sons. There are provisions for apprentice hunters before a person receives his/her hunter safety card.
Also follow the other advice above about info from G and F. If you sign up for mailings you will receive notice of classes and other information. It is most common to find classes in winter and spring but that does nut mean someone in your area won't offer a class for late summer and early fall. Keep watch on the G and F website.