Perform the following tasks to obtain the unique armors:
Gannon Family Tesla Armor The Gannon Family Tesla Armor is acquired during the personal quest for Arcade Gannon. When doing his quest and gathering all his allies, convince him to stay in Freeside for the duration of the battle. You need a VERY high speech ability -- at least 80 speech. As a reward, he will give you the Gannon Family Tesla Armor, helmet and all. You need power armor training to wear this.
Chinese Stealth Armor The Chinese stealth armor is located in the Hoover Dam offices. It's in the room with the toxic barrels and green goop. Once you see the room, turn to your left. You should see several wooden crates. Inside one of these crates is two sets of Chinese Stealth Armor. Both weigh 20.00. Unfortunately, the invisibility thing the armor does in Fallout 3, is gone now -- all it does is give you +5 sneak.
T-51b Power Armor Two sets of these armors a re located near HELIOS 1 . From there, go west towards the cliffs. From the cliffs, turn left and head to a small canyon, which is full of scorpions, that leads to a "Scorpion gulch". There you have 2 options up left or up right. Up left is Brotherhood of Steels camp, but we head up right. When you detect small amounts of radiation you're close. Kill a couple centaurs that are leaching the radiation and you see a possible meteor crater with large amounts of radiation. At the bottom you see two(2) dead Brotherhood of Steel Paladins who wear T-51b Power Armors. Armors are in pretty bad shape, but after repairing them you own possibly best armor of the game.
T-51b Power Armor (Non Faction) This armor does not have any faction effects and is the only NON-BROTHERHOOD T-51b power armor in the Game. The Deathclaw Promontory is located across the river East of Ranger Station Echo. Note that there is no map marker for this location. Look for a walkway that goes up a mountain right off the shore. You must defeat 20-25 Deathclaws, including 2 Mothers and 1 Alpha male. There are 2 dead prospectors bodies lying on the ground on the far right side of this area. One of them has the T-51b Power armor and a Tri-Beam Laser Rifle and the other has the Remnants Power Armor and a Multiplas Rifle.