Sometimes it is just the luck of the draw. I'm a bargain shopper. If I really am hunting for something, I do a search on Yahoo Auctions and Ebay.
Or once you get started, list on this trading board what you need and what you have to trade. It is hard getting your collection started. But we have all been there and understand!
If you know of anyone going to any of the parks, you can ask them to keep their eyes open. Maybe they can make a trade for you with someone there. And then in return you can trade with them.
Pinpics has a new feature that is wonderful. You can search for a pin.
For example:
Pin 2170: WDW Wedding Cinderella & Prince July 2000, shows 61 trading and 25 wanting this pin.
Click on: Members Trading. You can see who wants to trade.
Or try your luck on Seeing what you need to get this pin through a trade.
But sometimes it's easier just to buy the pin. ;-)