You'll find Dia in the south apartments, in a locked room near where you first awoke on Tarsis. She had a bit of a run-in with a man named Holdan; he drunkenly attempted to make an unwelcome pass at her, and she felt it necessary to teach him a lesson with a vibroblade. Unfortunately, Holdan couldn't take such a spurning lightly: he decided to place a death mark on her head, and now she fears for her safety and refuses to leave her rooms.
Light side Solution: Track down Holdan in the Cantina in the Lower City. You can attempt to talk him out of the whole bounty idea with a high persuasion, or simply pay the whole 200 credits that he demands.
Dark Side Solution: Congratulations, you just became a bounty hunter. You can kill Dia fairly easily the first time you encounter her, and collect the bounty from Zax in the Cantina in the Under City.
Largo is hiding away in the Upper North apartments. He got into debt with Davik and is unfortunately (for him) unable to pay it off.
Light side Solution: Give Largo 200 credits to clear his debt.
Dark Side Solution: Largo becomes Prestissimo for a moment, and then...the orchestra is silent. Return to Zax for the bounty.
Matrik is squirreled away in the apartments in the Lower City across from the Upper City elevator. He betrayed some of Davik's friends to the government for some reason, and now resides in a locked room, ever fearful for the day when a bounty hunter comes knocking at his door.
Light side Solution: Return to Kebla Yurt's shop in the Upper City (it's easiest to just warp back to your apartment first) and buy a permacrete detonator. Give it to Matrik for a decent experience boost and some Light side points, and then you can get the bounty from Zax.
Dark Side Solution: Actually, you don't seem to get Dark side points for killing Matrik. You can still kill him, of course; he has a mesh underlay on him that can be used to upgrade the Echeni fiber armor.
Selven's hiding out in the northwestern apartments in the Lower City. Since this is an official government contract, you gain neither Light side nor Dark side points for killing her.
Bendak's bounty is only collectible if you proceed through the five rounds of the Duel Ring side-quest, after which he'll challenge you to an old-school, mano-a-mano deathmatch. Participating in the deathmatch itself is an automatic Dark side shift, but winning it gets you 700 credits from the bout's promoter, plus the bounty credit reward, plus Bendak's blaster.