There isn't one, there are several well-known dealers of fine firearms that you don't see in gun stores, but which are often purchased by collectors.

https://simpsonltd.com/

https://shop.joesalter.com/

https://www.champlinarms.com/

https://hallowellco.com/

https://phoenixinvestmentarms.com/

Simpson's deals in lots of firearms, and many are not the super high-priced guns of collectors' markets, they're just guns you don't commonly find in gun stores today.

The next three on that list will be collector, high-$$$ guns, including "best gun" firearms. The last is a web site specializing in German/Swiss Lugers, which are very collectable.

Many dealers in collectable, high-value firearms won't deal in everything. It is very difficult to know everything about just English shotguns (for example). To know everything about high-$$$ English/Scottish shotguns, and then know everything about Lugers (as an example of two very collectable sectors of the firearms market)? It's not really humanly possible, there's just too much information to keep in one's head.