Two suggestions: Networking and Patience.
When first bought a pin, I didn't know anyone else in the region. I also was impatient. As a result, I ended up buying a pin on eBay from Canada and paying dearly to have it shipped to Florida...
Don't do what I did. Get to know pinball owners in your region. Go to events, auctions, join local/regional groups and forums. After building your personal pinball network, opprotunities to buy will begin to arise. It'll still be difficult to locate a specific title at a specific time... you'll need to either wait patiently, or be flexible with your desires.