Colossus. You get it by getting the rich achievement then going into the area Wrex is in in the Normandy then going to the C-sec requisitions officer (he's the green dot thing on the map) and then saving you game before you talk to him. After you save talk to him and ask to see his stock. If he doesn't have colossus or the kind of Colossus you want then reload your game and look at his stock until he has it. it may take awhile and you may be able to do it with other merchants (but you'd still need the Rich achievement) but it is by far the fastest way to get Colossus. Oh, and I'm pretty sure that you must be at least lv.40. Hope this helps!