Probably IRL - hobbyist groups, record fairs, things like that. I get the impression that the audience for this stuff is quite old and not as internet-savvy.
A word of warning, if you want to start collecting 1920s 78 rpm records (and the machines to play them) then that is a money, space, and time sink in and of itself. It's not like collecting vinyl (that's mainstream in comparison!), it's a world of its own.