Apparently DestinyDB has a list of other Ghost Shells - five more than the Generalist and Frontier people had at launch. One looks as though it would be something the Exo Stranger would have, whilst the rest appear to be from the Shipright, Vanguard, Crucible, and that arched door opposite the Speaker's Observatory we can't access yet...
It sounds as though some of these shells will be made available when House of Wolves launches.
Well, the Intrusion shell was known to be found in King's Watch, because people glitched their way into there. No doubt it'll be made available, once King's Watch opens during HoW. Hard to say how you will get the other ones.
There was a question during the twitch stream yesterday about shells, and one of the guys got very cagey. I believe more will be revealed in the coming weeks on that score.