Also, at the Water Merchants in the Hub, there is a locked room in the northwest corner where the Children of the Cathedral are located, that can be picked. On the shelf of a bookcase Rad-X as well as RadAway can be found.
In the Boneyard Library.
Vault 12 second level upper left room with a locker.
The effects of Rad-X are stackable up to two subsequent doses, but taking an immediate third dose of Rad-X has no effect. This is due to the fact that in the original Fallout, taking two doses of Rad-X granted 100% radiation resistance, enough to survive the most irradiated place on the map for several hours.