Below is a list of locations where you can find T-60 Power Armor set or parts:

• National Guard Training Yard -- Inside one of the outside storage rooms, locked by a terminal and guarded by some feral ghouls. (Note: As you leave, a Sentry Bot will attack you)
• Buy (or steal) parts from Teagan on The Prydwen.
  (You can get one full set for free if you do enough favors and become Knighted after joining The Brotherhood of Steel.)
• Buy (or steal) parts from Rowdy at the Atom Cats Garage after joining The Atom cats.
  (You can only buy from Rowdy after doing some favors for them.)
• Loot from the body of Elder Maxson in the quest Airship Down. (The suit has Brotherhood of Steel paint and is pre-modded)
• Outside the Federal Ration Stockpile in a trailer.
• In the back of atom cats territory there is a full set missing a helmet. Steal it while hidden and fast travel to sanctuary. Then buy the helmet from Rowdy.

Levelled spots:

What Power Armor you find at these locations depends on your Level:

• In a small secured area at the back of South Boston Military Checkpoint. A terminal (expert) needs to be hacked to have access.
• Quincy Police Station -- On Tessa
• Abandoned Shack
• Fiddlers Green Trailer Estates -- Inside a yellow trailer (with an orange line on top) to the North of the office building. (Hack the terminal to gain access)
• Somerville Place
• Mass Pike Interchange
• Revere Satellite Array
• Fort Strong
• Postal Square