Maybe you can get a lead on something like the yesterdaystractors.com forum. Or not sure if there is a forum or webpage on thresheree/old sawmill clubs that go beyond local in your area. Gotta cast a big net. Stuff like that has unfortunately been snatched up by the scrappers and turned into toaster ovens.
Edit: or perhaps you can try to contact these guys flywheel-supply.com. Probably have to send an email as you can go to their menu and select pulleys and it brings up pics and prices for a bunch of old flat belt pulleys but no good way of searching for sizes.