Where can I find (droid armor)?

You can take up cybertech and just make his stuff yourself. You can do side-quests and choose his equipment as rewards, or you can buy off the GTM. You can get orange (customizable) parts (like parts,motor,sensor, etc) from the planet commendations vendors on the fleet

Pretty much any missions you do early on will offer your droid parts also the things butterball said are also correct I would like to advise aginst buying off the GTM (galatic trade market) as any decent ones on there you can get from questing, drops, or making yourself. You should not need to get any better for your droid faster than you are doing your quests.