Having amassed thousands of discs for little outlay often given free. One way is to make it known to local vinyl record dealers/car booters you will buy 78's as they often refuse offers of 78's as they think there are no customers. In one case the record dealer just gave customers my phone number.
The other way is local free ads and just getting a reputation as that odd beardy type who collects 78's and they will turn up on your doorstep.
Caution ! This will also mean you will have more Bing, Kunz and Belafonte discs than any human would wish for.