I just can't seem to find a Stealth Boy and I need to do some sneaking.
No NPC sells Stealth Boys, and they are rarer than in Fallout 3. You may find some in locations with Nightkin (in their corpses or around the area). Here's a list with some locations where you can find Stealth Boys.
@Biff MaGriff I did the 'Come Fly With Me' mission near Novac, that involved many Nightskin, and I got a lot of Stealth Boys. Go check this mission.– DrakeMay 20, 2011 at 7:27
Dang I guess I shouldn't have sold so many to the gun runners :( May 20, 2011 at 16:35
@BiffMaGriff I learned that lesson the hard way as well. Sold a couple of them, I didn't use them much, then came to steal from the Gun Runner's and needed one... didn't have one. I had to go running around the mojave looking for one. Eventually succeeded. Jun 9, 2013 at 5:11
There is a stealth boy in the school house in Goodspring safe also