WDI pins are first sold in store at Mickey's of Glendale only to cast members. If they have leftovers they become available online to cast members, or cast members can bring guests into Mickey's of Glendale. The cheapest way to get them would be to trade for them, but most pins get in the hands of re-sellers and get re-sold for crazy marked-up prices. Unfortunately, most people I have met would much rather sell than trade, and many people are willing to buy just because it's easier to obtain a pin that way. Trading can be difficult because many who obtain these pins want to trade for really hard to find or valuable pins they want. Now, not all WDI pins are super hard to get, it just really depends on the design and how popular is the featured film or character.
I have been able to trade for some WDI pins I really wanted, though not so much for profiles or clusters, that's a different story...
On the other hand, if you are a D23 Gold member or know someone who is, you can attend Mickey's of Glendale pop up shops at least a couple times a year. There are also WDI pins sold at D23 Expo, but that is only every 2 years...