First off, you will not find any free cabs in the bay area. This is not the rural mid-west. Second, with a lot of unemployeed collectors, they will sell, but usually want a lot because they need the money. You best bet is to buy one from a local arcade or laundry-mat that isn't doing well financially. You can sometimes offer them cash on the spot and just drive away with it.
I've had pretty good luck buying from local arcades. You need to understand which cabs aren't making them money (that would be good candidates for Maming...typically older jamma cabs with crappy games in them). The arcade in the Valco mall in Cupertino puts their entire inventory up for sale to the public once a year (just before xmas). Most of the cabs are over $1K, but they have some older jamma cabs that sometimes go down to $199-299. They sold off a lot of these the last 2 years (I was lucky to get a 4-player NBA Jam). I've also helped a friend buy a cab from a local Golfland. The point is, you have to go out and around to find the deals.