How to get sith armour without mods

• To keep the sith armour you get on taris after you speak to Gadon, equip carth with the armour and remove him from your party.
  
• Then speak to Gadon him and pick the right choices to continue the game and agree to give him the armour.
  
• Then add Carth back to your party and he will have the armour equiped.

*Sidenote if you leave Taris without the armour equiped it may disappear, so make sure you or one of your party members are wearing it up to when you leave dantooine. ( Or to be safe give it to a party member you will not use a lot and simply unequip it from them when you want to wear it. )