Society of London Art Dealers was created in 1932.
Antique Tribal Art Dealers Association was created in 1988.
Copley Society of Art was created in 1879.
Society for Sanity in Art was created in 1936.
Fine Art Society was created in 1876.
Society for Art of Imagination was created in 1961.
Society for Art History in Switzerland was created in 1880.
Professional Amigos of Comic Art Society was created in 1995.
Art Gallery of the Society of Macedonian Studies was created in 1975.
London 1950's after WW2
United States Society for Education through Art was created in 1977.
Art dealers can be found in Portland at the PADA Oregon website. This website lists all of the main art galleries in Oregon, as well as events and news.
It depends if you want modern or older ones. If you want specific ones by specific artists, then that will be a lot harder, as you will have to look at auctions, private dealers etc. If you want modern works, then go to an art fair or an art dealer. For example, in London, there is the 'London Art Fair' which has thousands of works in every style imaginable, and all for sale. You could also go to a dealer in the city and see what they have to offer. Look up dealers online in your city.