Now that's a clickbait title if I ever saw one!
Friends, Family & Neighbors!
Everyone knows people, and most of them do remodeling to the place they live once in a while...If you let them know you are looking for locks, they are usually happy to hand over their old "Trash"!
This one is a bit harder since they have no incentive to do anything for us, so a little bribery usually works! Showing up with a pizza and an empty bucket at around 11AM works sometimes...others are happy to point to a trash can in the back room and say "Have at it".
Fair Warning - Unfortunately, some locksmiths seem to react with fear\anger as if we are taking money from them in some way...I simply Smile and Say "Thanks Anyway" then LEAVE THE PIZZA anyway. There is no point to antagonizing anyone by trying to change their mind.
Maintenance Technicians for Apartments\Condos\Strip Mall\U-Stor-It\LaundroMat\etc.
This can be a gold mine in practice locks for a club! These types of techs usually end up with cut or otherwise "useless-to-them" locks. I usually either offer to help them personally with something as a trade, or go with the Pizza and bucket trick.