However, since companions can wear power armor, you may want to deck them all out and will need at least two.
If your goal, like mine, is to have a Power Armor dealership at your disposal then read on. A couple of things to note though: the armor on the suits is determined by level. It might behoove you to discover them later. It might not.
If you know of other locations, please provide info in the comments and I will add them to the guide. If you are looking for the new Fallout 4 Far Harbor DLC power armor check out our Far Harbor Power Armor Guide.
How to Find Quest Related Power Armors
First: Your first suit is found early on in the main quest when you rescue the Minutemen.
Second: Second quest related suit of power armor is awarded by the Brotherhood of Steel.
In the World
Location 2: North of Fort Strong at a Checkpoint (in the building)
Location 3: East of Fort Hagen in a Trailer Park
You will need advanced lockpick skills for this.
The suit is in the water just about where the targeting dot is in the image below.
Location 5: Upper Deck of Highway East of Greygarden
There is an elevator at one end of the overpass just north west of College Square.
It looks like this:
And is here on the map:
Go up the elevator and to the left towards the crashed Vertibird and you will find a frame and armor:
Here on the map:
Enjoy your power armor hunting! There are quite a few more that I have not found yet. If you know of their location, please comment and provide details so I can add them!