I own a locksmith company and we do a lot of bank service work (and have for many years) so I have become good friends with many employee's at many banks.
Normally I just ask when I am working at one of the banks what they have or if I am working inside the main vault, I'll just glance around.
Bankers talk to each other and my friends ask around other banks for me to see if they have any halves and call me when they find some.