Is there clean water for sale in Megaton?

I'm new to Fallout 3, and on arriving at Megaton, talked to a beggar outside asking for clean water, which I didn't have (only slightly radioactive). He said they had clean water for sale in Megaton. I said I'd get him some. I spent two days of game time in Megaton, and probably explored 80% of it, making friends with various people including the water purifier engineer and the stores, saloon, and restaurant shopkeepers, talking to almost everyone (the clinician too), and looking at almost every bottle and cup and water source... I didn't see any clean water anywhere.

Was the beggar just wrong that there's water for sale there?

If you do the bomb disarming job in Megaton, you can get a house there. In that house, you'll find a robot* who gives you a few bottles for free every day.

But, in my opinion, blowing up the atomic bomb is FAR more satisfying. Screw your karma.

Fresh water can be obtain randomly from random merchants who sell food. In Megaton there is one near the bomb.

Usually you find fresh water. It spawns randomly in cabinets and lockers. There are no merchants which always sell water. It's a lucky drop more or less.

_{*You get a robot butler in the Tenpenny Suite too. So it doesn't matter if you blow up Megaton up or not.}

The beggar is nothing but a hopeful; Megaton does sell water, although it is irradiated...as you increase farther into the game, purified water will become more and more common, and you may return it to him for karma.