Pinball is my hobby, and I know lot's of people in our hobby -- games come up for sale, from time to time.
e-mail me at pinscott@gmail.com, and I will see if I can help you.
It would help to know if you have a budget in mind, on what you are looking to spend. There is a big differences in what the games will cost.
Here are some starting points, but don't rush into this.
1. Talk with pinball people near you. Getting to know the local pinball people is the best.
2. search -- http://user.xmission.com/~daina/classified/index.html
3. join the free pinball forums, like - Michigan Pinball League - http://groups.google.com/group/Michigan-Pinball-League?hl=en
4. Attend Pinball At the Zoo, Kalzamzoo, Mich - http://www.pinballatthezoo.com/
5. Attend the Michigan Pinball Expo, Rochester, MI - http://www.mipinball.com/
6. Attend the Ohio Pinball Show - near Akron, OH - http://ohiopinballshow.com/
7. Your chances are low, but you can search Craiglist - this is overly watched and hard to find a game here
8. Only buy from an e-bay seller if you can drive and see the machine in person before bidding.
9. If you are are a hobbiest, you may not likely need to purchase from a retail store, or one of the online website store. This will depend on your ability to fix and repair things.