I suppose you could find some insurance company, somewhere, that would underwrite anything, but I can't imagine the cost would be reasonable. Also, I'd suggest finding another way to describe what you plan to do - do you really want to have to face a civil suit at some point and explain why you selected the "wrong" firearm for the plaintiff? Knowing that anybody can be sued by anybody for anything, I think I'd find a less culpable sounding line of business. The mere fact that you think you need insurance for this line of work tells me that you're worried about some of the same things I am when I think about how you've described your business. It might be better to describe it as a "Security Consultant" or a "Personal Protection Class" or some such. Always have your clients sign waivers saying they have received professional training elsewhere and are well versed in firearms safety and local laws, etc.
Just my nervous $0.02.