It's all about keeping eyes and ears open. When I drive anywhere here, I look for pizza places that are closing, or peoples barns/garages when I pass houses. Also helps to work at an arcade where people are constantly coming in with "oh, I saw one of these at blahblahblah"... I'm too broke to follow up any "freebies" as my last couple of freebies cost me a small fortune (Dragon's Lair) and headaches (BattleZone, PolePosition)... but every once and a while... a good deal is actually a good deal... rarely, but it does happen. My Tron cabinet was given to me empty and I just stumbled/worked off everything I needed. I think I have about $90-100 total in that machine. Same with Robotron, free cabinet, $60 for complete working boardset. @$30 for a REALLY nice Taito cabinet with Empire City in it. $100 Ms. Pac mini that just needed a cap kit. $100 HUO Route 16 cocktail... $80 Space Invaders that just needed some caps replaced. "Free" Asteroids Deluxe, Time Pilot, AstroCity Blast cabinet, Gyruss cab, etc etc.
I guess it's all about luck, but you have to work on getting luck to work for you. Make all nice-nice with local operators and arcades (non-corporate arcades, we give away machines all the time to people that help us). Ask them if they need any help from time to time with working on things... Eventually you'll start raking in the machines. I promise you that they have dozens and dozens in storage that they simple can't put on location (older machines that just won't make the money needed for repairs). When you get in good... roll the dice and mention them. You might just be surprised.