As others have mentioned- flea markets, swap meets, yard/garage sales, etc.
But you really have to put the effort in, and have good "people skills". Don't just go by what's on display, you have to ask the person there if they have any old knives.
And it could be the person you least expect who has a knife or two. It might be some little old lady sitting there knitting with a table covered in Hummel figurines and other knick-knackery, but if you ask her about old knives she might pull out an old cigar box full of them.
I'm a member of a switchblade forum and some guys go to such places to find old switchblades, and not only do they sometimes find real treasures, but as I mentioned, it can be the seller you least expect.
Pawn shops can be another source. Sometimes when a family member dies and a person doesn't know what to do with their knives they just take them to a pawn shop and sell them.
And then there are family and friends. It's not uncommon for men of past generations to have a few old knives hanging around in an old tool box or tackle box, etc. And if they're not around anymore, and their loved ones have no interest in the knives, they might be willing to part with them.