I've been collecting for 30 years now. In the beginning, and keep in mind this is
pre-internet when the big 3 manufacturers were still around, I'd often get
1 or two a week. Mostly via classified ads in the local paper. The other way
I'd get pins is from other collectors who could not fix machines they bought.
Depending on which machines were involved I'd get one pin working in
exchange for one project. Back then it was not unusual to find several
pins in one garage or storage unit.

However the past few years its been more like 1 every couple of
years. Reached saturation long ago so a pin needs to be a killer
deal or one I really want.
Steve