I think the OP is talking about “window shopping” primarily. Looking at the old guns and gear for enjoyment and education.
I like the Rock Island Auction, Cowans, and iCollectors, all of which have good descriptions and uniformly excellent photos. However, I am usually less likely to go to some specific seller’s website than I am to just do a Google search, using terms like “antique hunting pouch,” “buffalo powder horn,” “cane powder measure,” or “tack decorated Springfield.” Maybe only one out of twelve or fifteen “hits” show anything you really want to see, but if you get a thousand hits, you still find a lot to look at. Check out the “images” page in addition to the links. These frequently lead to one or more of the auction sites mentioned above.
Some of the folks curating these auctions don’t really know what they’re talking about… I’ve seen several “powder horns with missing base plugs” that were clearly blowing horns, for example… but the Rock Island and Cowan’s staff (and a couple of others) generally seem to know their stuff. I’ve learned a lot from that type of browsing.
Some of the larger museums (e.g. the Smithsonian, the Met, the National Museum of the American Indian) have digitized their collections and created searchable databases. A lot of these places have guns and related accoutrements, and they are worth a look.
Notchy Bob