There are 3 major companies.
http://www.corevensguninsurance.com/
http://www.historicfirearms.com/
http://www.collectinsure.com/
I started out with Historic Firearms. After a few years, I decided to change.
I had met the agent from CoreVens at the Colorado Gun Collectors Assn show a couple of year prior and he had been helpful (even though I told him I was not changing. So I went with CoreVens.
After a couple of years I made several inquiries to Collectibles Insure - both two different years at the SOS show, once at the Max show and even by telephone. What I wanted was two times: (1) a sample policy so that I could compare terms before I signed up. The devil is always in the details. Even though I asked several times, I was never able to get a sample copy of the policy from them. (2) I asked the question how their policy was the same as that of the other two companies and how their policy was different. I was never able to get them to tell me that. So I never had a policy with them. (The other companies answered both of them.)
I presently do not have such a policy. My annual premium was in excess of $3,000. My wife became sick and I had to put the money towards her. (Also ended up selling a few things also.)
The companies will insure guns, holster, stocks, etc.
They have different requirements for specific listings (depends on value) of firearms, safes, alarm systems, taking items out of the house, to gun shows, etc.
I would suggest that you make a list of your activities, etc which will help you in getting the proper coverage, etc. It can make a difference if you take things (and what things) out of the house.
With respect to value use replacement value. Try some of the dealers. Take good photos of your items cause if you have a claim, you will have to prove condition. (Too many people insure their doggy firearm as a diamond.)