(I'm not sure if this belongs here or in General.)
You only need 8, at which point, you get a new chest. I got mine basically as I caught the ship out of the zone, so it's more a parting gift. There's a vendor in the Temple cantina, and there's a vendor in the quest hub east of the Temple (the first one you get sent to). I haven't doublechecked that the vendors have exactly the same thing or not.
A lot of the cool stuff opens up at 16: I got a very shiny new lightsaber at 16. That said, almost everything else available, I had better.
When I was levelling up, I was wondering: take the upgrade, or take the commendation? My recommendation would be "take the upgrade". You can cash out commendations by buying lockboxes.
The vendors are in the west wing of Vendor Alley (the below-ground vendor area near the Senate tower), in the most southwest niche. Note that the guy in the center-back sells weapons which, at least for a Shadow, are not as cool as the weapon vendor standing behind the east counter. I'm pretty sure there are vendors up in the Fleet, but I haven't explored there.