To get this rare armor for all of your team mates, or even for just Shepard, is a feat. But not unmanagable. There's the right way, the random way, and the cheating way.
The Right Way:
Buy ONLY the license that belongs to the company that manufatcures the Colossus armor, which is the Kassa Fabrication license. This will make it more likely you will get the armor when you visit the Normandy Requisition Officer.
The Random Way:
Open every box you see and hope you get it. Very hard to do, but as you rise in levels, you'll increase your chances of getting it.
The Cheating Way:
It's called the famous Vendor Reload Trick. After visiting a plot world with a port like Feros or Noveria, complete the mission and visit the Requistion Officer aboard the Normandy. Save before you speak to him. If he does not carry the armor, reload to the save you just took and try again until you get it. This is also an easy way to get Wrex's very awesome Battlemaster, Rage, and Berserker armors. You must be level 36 to obtain both of those licenses, and neither come cheap.