Where can I find Moira Brown after blowing up Megaton?

Solution 1:

From Fallout Wikia

If you decide to blow up Megaton, she will not die; instead she will become ghoulified. If you started the Wasteland Survival Guide quest, or met her to receive an armored Vault jumpsuit, you will have to visit the Megaton Ruins to find her. If you hadn't started it, she can be found in the Underworld, whereupon she will claim she was 'out-of-town' during the detonation, and apparently unaware of her recent affliction. Depending on how you break the news to her, she will either fall into despair or remain positive. Eventually, she will leave for Rivet City and finally Underworld.

Solution 2:

I've had my own experience with her.

I just walked up to the rubble and she came out of no where. She didn't know she was a ghoul until I told her and after she said she was going to Rivet city.

After she goes to where all the Ghouls live the Underworld.

It matters how you tell her too. She nay be depressed or happy. And she says she had more excuses to travel the wasteland.