I have been a frequent visitor to
flea markets around Stockholm for several years and I have never found any mechanical keyboard. I collect other things than keyboards, otherwise I wouldn't go to as many.
Over five years, I have picked up maybe half a dozen rubber dome keyboards from 10 to 50 SEK each: Key Tronic, BTC, MS ergo, Cherry RS-6000. My best find was a HHKB Lite 2 for 50 SEK.
If the keyboard is dirty, you can usually haggle the price down - the seller would not know that you are going to clean it either way.
The closest to a mechanical I had come was when I met a guy that had half a dozen IBM rubber domes.
Me: "Hello. Do you have any mechanical IBM keyboard by any chance, one of those that makes a click sound when you press a key?"
Seller: "You mean those that make a lot of noise? No, I had but I got rid of those. I didn't think anyone wanted that old junk"
Me: "AAAAARGH!!!"
I have had better luck finding mechanicals at Tradera. I can see a couple of Cherry G80, an Alps and a clicky IBM keyboard there right now, but the IBM keyboard is overpriced and I expect there to be bidding wars on the Cherry keyboards.
The Cherry G80 series has Cherry MX switches while other Cherry-brand keyboards usually don't have mechanical switches.