Networking.
In the past couple months I have scored 4 DMD machines literally within walking distance of my house ... purely from word of mouth. A friend put me in touch with a business that had a neglected old game in their rest room. And a pinball friend doing a delivery hooked me up with someone local with other games to sell.
All of these I pounced on quickly, and after some negotiation got for a good price Having cash and being able to do a hassle free deal quickly is key ...before they advertise them on Ebay or other Classifieds.