The Remnants Bunker is accessed at the end of Arcade Gannon's quest line, after recruiting all the former Enclave members.

Brotherhood T-45d Power Armor: Found on a Paladin in Dead Wind Cave. This is unique as far as I have found, but looks identical to normal T-45d. Counts as a disguise.

Where is this cave and where is the paladin in the cave? Is he hard to find?

The cave is west of Ranger Station Charlie, it's very near an abandoned shack, so you'll know you're there if you find that. The Paladin isn't hard to find at all. It's really a matter of getting to his corpse that proves the challenge.

I recently found a suit of Enclave APA1 (ie Remnants Armor) sans helmet in the deathclaw promontory. A little north of Cottonwood on the opposite side of the river, there's a path up the mountain. Follow that up and head right. You'll encounter about a dozen deathclaws, and towards the end, you'll find two prospectors. One with a full set of T-51b armor, and one with the APA1 suit minus helmet. You'll also find a multiplas rifle and a tribeam in the area.