Once you have unlocked the characters, go into free play mode in any level. Make sure that you have Kit Fisto and The Emperor as two of your characters then kill The Emperor with Kit Fisto.
It is easier to go into a level than try to find them wandering around outside.
Edit: I believe Kit Fisto can be purchased for 35000 after completing Episode II level 4. The Emperor can be purchased for 275000 after completing Episode VI level 5.
Edit: Apparently codes can be entered for both characters. Thanks to xDeathFMx
for the info.
Code for Kit Fisto is: TFI888
Code for The Emperor is: EMP666