This article is from the Pinball FAQ, by Andy Oakland sao@REMOVETOREPLY.mit.edu with numerous contributions by others.
The path a pinball machine travels typically looks like this:
Manufacturer--->Distributor--->Operator--->Collector or junkyard
Unless you have very deep pockets, you won't be buying your machine
from the manufacturer or distributor. Operators are the ones who
put machines out in the field and maintain them...They're usually
willing to sell used machines once they stop pulling in the quarters.
Go to your favorite machine in the field, and ask who owns it. If the
location doesn't, there's probably a sticker on the machine pointing you
to the operator. Another way to find operators is to hit the Yellow Pages,
and call up the companies listed under "Amusement Devices." First ask
them if they sell machines for home use, then ask for the specific machines
you're looking for.
Part II of this faq also includes pointers to several sources for
used pinball machines. These are typically large operators.
You can also buy machines from collectors. In fact, this is pretty much
the only way to go to find an Electro-mechanical. You probably aren't
going to find an EM in the field, or with an operator.
For both EM's and solid-state machines, the little ads in periodicals like
Game Room are an excellent source of leads. (See list of periodicals
below) Also, you can try to find something locally. Buy every newspaper
you can, including the little "nickel ads" type, and check the classifieds.
Keep doing this for months. Takes time, but good deals occasionally pop up.
You can also find a "broker," a sort of super-collector in business
to buy up old used games, fix them up, and resell them. Again, you can
reach these people through the publications listed below.
Also, believe it or not, check with a dart supply store! I know of two
in my area (Boston) which sell used pins, and at least one Norwegian