When you're done with all the Story Mode levels, there will be a new door open in the Cantina, leading to Jabba's bar. In there, you can access 10 quick missions in which you take control of a squad of Bounty Hunters and your goal is to find some rebel leaders. The faster you manage to do that, the more you will earn. You also get a Gold Brick for completing each one of these.
Next to the sought character's name is the name of the level you're wandering around. Not to complicate things unnecessarily, I'm trying to relate to minikit locations in below descriptions. Just check the Freeplay section for details.
HINT: You must have either 4-LOM or IG-88 in your team on these levels, as they can use both R2 and C3PO panels. Buy them at the shop before you kick off.
R2D2 (Secret Plans)
R2 is where you've found minikit #5.
Obi-Wan (Through the Jundland Wastes)
In the last room of the Sandcrawler, just where you opened the exit as Obi-Wan in the Story Mode. Destroy the barrels to the right, Ben is right behind them.
Chewbacca (Mos Eisley Spaceport)
He's behind the left door near the Dewback fight arena. The door to this section can be blown up by a detonator.
Leia (Rescue the Princess)
She's in the second to last cell on the left in the prison block.
Ackbar (Escape from Echo Base)
Right where you've found minikit #8.
Yoda (The Battle of Endor)
Go to the left and use the turnstile, then use Boba Fett's jetpack to reach him.
C-3PO (Betrayal Over Bespin)
Right where minikit #6 was.
Lando (Betrayal Over Bespin)
At the control room at the end of the level (where you operate the bridge).
Luke (Mos Eisley Spaceport)
Right where minikit #6.
Han (Escape from Echo Base)
Right where minikit #2. In order to get to him, you have to turn on the carriage.