In The Hub at the Water Merchants in the house of the Children of the Cathedral, there is a locked room that can be picked. Inside that room, in a bookcase, Rad-X as well as RadAway can be found.
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 Glow (the most irradiated place on the map) for several hours.