This video explains how to unlock the ability to buy Han Solo. You must find 5 characters in order to unlock him.To find them, they are in these levels:
C3PO - Lost Temple
Luke Skywalker - Into the Mountains
Chewbacca - City of Danger
Leia - Free the Slaves
R2-D2 - Desert Ambush
After you have all five found, go to the Library and buy Han Solo for 100,000 studs!