...his information?

During the whole novel, DOn Corelone is the most informed person of the Corleone Family. Even Tom Hagen who is his auxiliary brain, is not as informed as the Don. This is strange because the Don rarely did something without consulting Hagen.For instance, the Don knew before everybody that Moe Greene has slapped Fredo in Vegas. So I was wondering where Don Corleone got his informations?

First, the Don was a lot older than Hagen, and by being around longer, he had a larger network of contacts.
Second, in the grand Sicilian tradition, he was a "personal" Don--that is, he was available and accessible to people, who probably called him or talked with him at every opportunity. Remember that he upbraids Bonasera thus: "I can't remember the last time you invited me to your house for a cup of coffee." Not idle talk: he probably would have gone to Bonasera's if Bonasera had invited him. So I have to assume that the Don had a constant stream of visitors who kept him up to speed.
Third, probably a lot of the old-timers among the Don's constituency didn't feel comfortable around Hagen, for obvious reasons.
Fourth, the Don probably encouraged all of these contacts as a way of constantly checking Hagen's loyalty as well as his judgment.

Thank you once again for your insight, Turnbull. But the Don is sick most of the novel and it was written that he never spoke on the phone in fear of him being set up by the feds. So when did he have time to be informed since he was in his bed and always consulted Hagen?

And if you remember that Fredo was under Don Francesco's protection, and that he was often informed on the well being of Fredo, it is not likely that Francesco called up anybody but Don Corleone to inform him about his son.

Yes. Fredo was actually being protected by 2 Dons. The one in L.A and San Francisco