I've killed Ceasar and I'm pretty sure the legion doesn't like me too much. I've heard if you side with them and get their safehouse that there is a dude who will give them to you. Everyone I've ever had I used to pickpocket my caps out of the dude at Mojave Outpost for fixing my gear. Is there anyone who sells them or a place to find a lot???

So far there are two long term supplies of stealth boys one is Caesar's Legion (From the guy at the safe house and random supply drops). And the other long term supply requires you owning Old world Blue's The X-13 Research Facility.

So it says on Wiki but it all comes down to luck for the most part they can randomly unlikely but found in ammo boxes and people selling them I have only ever gotten 4 random ones from boxes and 2 from people the great Khan armory had 2 they were selling not sure if this helps but good luck finding some.

I do have Old World Blues but I haven't started it yet. Is there a vendor for them? I just want to make sure I don't miss the opportunity.

I remember getting around 15-20 from the Repconn site near Novac

There are just a few around inside the building that re-spawn every so often i'm not sure about the times haven't played New Vegas in a while.