looking for flying fish
I am assuming you mean the whole fish, not the roe. The roe is available lots of places. The whole fish I've only ever seen at Mike's in the South Market.
But, tide yourself over until you find some to cook yourself by having the grilled flying fish at Kon-Nichi-Wa on Balwin. Its excellent and fairly cheap (10ish for a good serving). The woman who runs the place is really friendly, you could even ask her where they get theirs.