After you talk with Voodoo Guy about Bob Olmstein ? you would need to get to Fishmouth. Last time they spoke Bob was staying at the Finman House.
Just talk about Necronomicon and what could help you with that until VG mentions Bob Olmstein.
With the taxi, of course. Talk to the driver. Offer to pay double. Offer everything you got.
Help him with that.
Nice. Go talk to the taxi driver. He will now drive you to Fishmouth.