They are available at different locations- I think it varies for each pin. Usually the main pin trading locations at each park, Downtown Disney, and at the DVC resorts. You just have to show your DVC membership card. You may have to ask at the counter for them- sometimes they aren't on display.
Also, stop into the DVC sales kiosks and look on their lanyards. Last week I was able to trade for the Goofy-Best Kept Secret one and the Members are Magical Sorcerer's Hat, both on the lanyards at the DVC Preview/Sales centers.