Advanced radiation suit (Fallout 3)

The advanced radiation suit is a suit specially designed to stop radiation better than the standard radiation suit, meaning it has the highest amount of radiation resistance of any armor in the game. It covers the entire body. As such, it is impossible to wear a helmet or glasses with this armor, except the biker goggles.

Fallout 3
It can be found deep within Vault 87, in the Test Labs. When you get to the room with the Lab Technician's terminal (on a desk to the right) one of the options will be to unlock the storage room. Choose that option, then turn around and go back out the door you entered and turn left. That door is the storage room and contains the Advanced Radiation Suit.

Another may be found in the Franklin Metro Utility, in a box titled "Radiation Suit Case", at the bottom of a flooded tunnel.

One can also be found in Point Lookout, inside a muck hole during An Antique Land.

Another 2 are in Mothership Zeta, in the operating room in the Cryo Labs, on a table.

Can also be bought from Moira at Craterside Supply, Gustavo or Michael Masters from Tenpenny Tower, Karl at Meresti Metro Station, Tulip in the Underworld, Flak 'N Shrapnel's in Rivet City, Crow if you invest the maximum amount into his inventory and Knight Captain Durga in the Citadel if you get permission to trade.

Fallout: New Vegas
In Fallout: New Vegas there is an advanced radiation suit just south of the Nipton Road Pit Stop in Crescent Canyon East. The entrance to the canyon is found near the very bottom of the map, and directly in between the Nipton Road Pit Stop and Nipton itself. Note that there is a Radscorpion nest in this area, containing several Giant Radscorpions. Following the rail track from Nipton bypasses the Radscorpion nest. Once in the canyon head west, following the canyon floor. You will pass underneath the remains of an underpass, through an area of wrecked train cars, radiation barrels, and radiation pools. Eventually you will find a blue semi-trailer turned over in the mud (if you are facing west in the canyon, the trailer will be on your right). Next to the trailer, also partially buried in the mud, is a silver radiation suit case.

Two advanced radiation suits can also be bought from Quartermaster Bardon at Hoover Dam Offices if you have speech 50+ and adequate NCR rep.

Appearances
The advanced radiation suit appears only in Fallout 3 and Fallout: New Vegas.