My recommendation for file dumpster-diving is to seek out one of the few-remaining old-time hardware stores or an old-time store that grew into an industrial supplier and check their file inventory, looking for old and dusty ones in the bottom of the pile or the back of the hanging hook. Files aren't a big seller any more.
I did that and was able to find eight or ten old Nicholsons (various cuts and sizes) from back when they were made in the USA and bought them all. They were in the back, covered with dust and grease, obviously ignored by buyers for the clean ones in the front of the hook or pile. Yes, their stockers didn't believe in rotating stock, much to my benefit.
One even more elderly gent than I working there saw me come up with my assortment, smiled at me and said "good choice!"
It worked for me; it might work for you.
Jim in Alaska