Most likely Little Green Men and/or DIsney Pin Trader. I'll be there to get it! Other Limited Editions may include RSP or wristband distributions, ODPT is the best way to find out about each individual pin you are interested in. Hope that helps!
Check out the Official Disney Pin Trading website. They have information on each pin, including price, release date, and what store to buy it at. If there are special instructions for a release (like a Random Selection Process) there will be details there.