I'm not aware that it is offered as an on-line class. I know here in CT the Connecticut Fire Academy offers it. Essentially, it teaches you how to teach, something I suspect would be difficult to do online, as each student has to make a required class presentation on an assigned topic. But I could be wrong.
I've never seen this offered as an online course either. While some of the classroom stuff could be covered online . . . it seems kinda oxymoronic to want to become and instructor and teach others and then taking an inpersonal online course to become an instructor.
And as Jack said, there is the class presentation on a subject matter to do at the end that just could not be done online.
MS Fire Academy offers it through correspondence. They send you the course materials and you have 6 months to complete it. As far as the presentation goes, you record yourself delivering your lesson plan and mail it in. However, I think you have to goto the campus to take your tests. Not sure though. Further info here...