Ebay is a starting point but it is a minefield. The best way to find them is people you know. In the 50s 60s and 70s everyone had mechanical watches (well almost the 70s). Your grandfather, uncle, or other distant relative may well have one in a drawer. Ask friends' parents and family friends. My Constellation Pie-Pan was from a casual acquaintance that happened to collect old Omegas (he never wore it, just kept them). I bought the Pie-Pan for about $850 or so, paid $150 to get some minor work done and regulation, and paid $120 or so for a brand new bracelet. The result is a very cool vintage Omega.
Very recently, I was looking on facebook and noticed a friend's dad in a picture, wearing a watch I thought looked like a vintage Omega automatic diver. I contacted him and we talked it over, turned out my first instinct was correct, the watch was an Omega Seamaster 120M "Baby PloProf" automatic diver. This mate's dad had gotten it in the early 90s from his father and had worn it ever since, probably not realizing what he had on his hands. I let him know about it and what to do to keep it in good condition as well as giving him a rough ballpark value. I didn't offer to buy the watch as I normally would as it no doubt meant something to the family and should stay with them but I now know that a valuable Omega has been saved, and will be cared for, and that really is all that matters.
They're around, thousands of them, in drawers, on the wrists of people that just consider them to be old watches, and owned by people you don't think would ever own a high end swiss watch. The fun part is finding them
