Any insurance company can offer you a "rider" that will cover your collection. The premium will depend upon the value of your collection.
Many variables to consider as well, when it comes to determining the premium. Things like where you live, your type of house (construction), any alarms, is your collection in a safe or vault, etc. And, the insurance company will want pictures and serial #'s of each firearm, and most likely, a professional appraisal.
I myself, have an extensive collection. Because of specifics that I have taken, I haven't insured it.