Of course, everyone here knows of Barber. I came across two via an unrelated topic search - a web page showed an event being hosted at a motorcycle museum in the UK: http://www.nationalmotorcyclemuseum.co.uk/ A Google search for the above also listed this site in Iowa (Iowa?): https://nationalmcmuseum.org/ What others are there?
https://allenmuseum.com/ Long story how I ended up at this guys place but was there before he was a museum so to speak. Just "some guy next door to my aunt has this cool collection". It's behind his house from what I remember and he rents the place for private events. Really nice guy.
I was going to suggest the museum on the Isle of Man. I couldn’t remember the name so I googled it, there are at least 3 on the IOM. I figure there is reason enough to go back I missed 2
There's a small one here in Carmel CA http://www.mototalbott.com/ They have a Proton KR5 --- Ducati Museum in Bologna Italy https://www.ducati.com/ww/en/ducati-museum-and-factory/museum I've been to both.
https://billsbikebarn.com/ Bill's an H-D guy. Prolly not anyone's "special" on display, but there's at least a couple hours of stuff to look at if you pay attention and it's not really H-D dominant nor "bike only". Memorabilia, other devices/vehicles, etc...kinda just a place a motorhead could appreciate, but bike/motorama/Americana related.
While compared to Barber (and as far as overall space goes) Talbot is small, there are a heck of a lot of great bikes there. Not sure on count, but I'd say 120 or so, and they are very, very nice. In addition to the proton, there is KRJR's and Wayne winning smokers. If you are in the area, highly recommended.
+1 Pretty Anglo-centric, but that's to be expected. Sammy was there working when I went and stopped to have a chat.