The alien crash site is
north of Minefield, and a bit further north than the MDPL-13 Power Station. You'll know when you're close because you pick up a radio signal from it. The ammo for the Alien Blaster is very rare since it's so powerful, but I believe you can find more in a limited number of locations (including Fort Constantine, I am informed). I also hear that occasionally a random encounter pops up which consists of a loud boom. This is supposed to be another ship crashing nearby, but I didn't know this at the time so I never really looked around.
The awesome experimental armour is found in
Old Olney. It's a ruined town to the NE of the world map, East of Vault 92 (within spitting distance, practically). A large grate in the North end of town will collapse if you stand on it, with the only way out being through the sewers. There's a few Deathclaws inside, and near the end you'll find a dead Brotherhood initiate surrounded by corpses. The initiate is wearing the armour, but it isn't pleasant (same voice as a Mr. Gutsy*). Also, although it's a side tunnel you find the armour in, follow it to the end for a copy of "Duck and Cover!".
Your final spoiler (technically the first) can be found in
Oasis, to the far North, where Bob (or Herbert) has overtaken Harold and rooted him to the ground. You'll need to talk to the Tree Worshippers in the area to actually gain access to Harold, though. There's a pretty decent quest involved. To be honest, Oasis is a little tricky to really find, since you need to follow a cutting in the mountain up to it. You can get a map marker from some mercenaries who I found on a section of destroyed bridge to the East of Girdershade.
*
"Listen up, you God-Damn puke. You are now wearing Prototype Medic Power Armour. You keep me safe, and I'll keep you safe."
