1) Vito instructed all his sons about the "olive oil business". He must have spent a lot of time with Sonny, Tom and Michael, discussing all that stuff. But it's not difficult to assume that at least he tried to transfer some of his knowledge to Fredo. This knowledge must have included a number of his contacts, especially contacts to old friends and partners.
2) Fredo had been in Las Vegas for more than ten years. He had known Moe Greene. It seems improbable that he didn't get to know Roth and Ola during that time.

Anyway, it seems to me, that again we split in to parties: One tries to find holes in the script. The other one tries to defend it against all improbabilities.