This group sells information to Mossad agents. I am wondering if this organization has its real world counterpart or just a convenient way to advance movie plot?
Short answer: No, the shadow organisation is not real. Its entirely fictional. No source on internet nor books writing back it up. Information were gathered from many sources: Mossad field agents, western intelligent agencies and even private investigators. Those characters vaguely cover the real use of private investigators, information sellers.
Longer analysis: I came to similar conclusion what you proposed, that it is just convenient way to picture much complex situation.
It can be argued, that Spielberg might have had incentives to not show on screen how the actual information were gathered. Western countries would not like to see themselves in light of how they helped to assassinate people in 70s. Also it is not in intelligent agencies benefit to spread how they work internally. The book is critiqued and discouraged by these points. It may have influenced Spielberg to leave that part out.
Also another argument I deduce from Spielberg interview is the themes of human motivation and empathy alongside of violence. The shadow organisation key persons are portrayed as warm family persons, while they actually sell kill-list information to who-ever pays most. Those characters fit to what Spielberg wants to portray. And such application might have been hard to put into practice, when multiple characters/organisations would have been used.
But I do not have verification for this. Just what I have deduced from sources what I have found.
As this questions has not been closed, I will answer it from my question here. Also all sources are listed there.