I suggest you check the auction house for mod-able heavy armor, even for higher levels.
Additional, you can check standard heavy armor vendors on the different planets (usually at the starting area), sometimes they sell instead of a white item a mod-able chest, or check the commendation vendors, they often sell mod-able armor pieces.
Also HC2/4 Quests sometimes give mod-able equipment as reward.
Perhaps also just roam the republic fleet/station, and look at the equipment from other players. If you find a player using a nice chest piece you would like to use, then inspect him, and check for the item in the chest slot. Then google the item's name e.g. www.torhead.com
, and see where you can obtain it.
I hope they will add a remove/show hood toggle, some of the jedi knight armor pieces look nice, if they won't have the hood showing...