Networking is key.
Don't be afraid to mention the hobby when you meet new people. Some people might think it strange (f--- them), but a bunch will find it cool. Better yet, some will know of machines that have been in basements, garages, storerooms, etc. of friends and family and will gladly offer that information up. Found my Funhouse, Swords of Fury, and just last week my Tapper (the one I used to play on location!!) that way. Also got in to a few operator stashes like that as well. Never hurts to mention it to people.
Get to know the collectors in your area. The cool ones will give you a heads up if they see a machine they know you are looking for. The ones that become your friends will put up the money to get that machine for you and bring it to their house.
On the subject of operator hordes/stashes/warehouses, they're still out there. At least around here. Obviously not as many as there were 5/10/15/20 years ago, but some second and third generation ops are still sitting on a bunch of stock. It's out there.