You find it all over the place. Just scavenge and look in EVERY box, EVERY locker and EVERY first aid kit (start here first). If you're willing to pay 100 caps, doctors will also cure you of radiation. Also, some of them will sell you Rad-away. Random vendors may not carry it always, but the various doctors likely will.
Also, take Rad-X and find a radiation suit if you're heading into heavily radioactive zones.
All you guys dumping on the space suit keep in mind that the weight and resistance is the same as the Adv radiation suit BUT, you can wear glasses and a hat with the space suit effectively giving you another 5 DT or even wear Boone's beret, like I do.