Not sure if it’s difficult to get Tuna Belly, but I see that it’s well known enough that there must be some way to finally find it in a fish store. It’s a very tender fish anyway. (My gourmet past allows me to say this on behalf of there having been other people interested in the purveyorship of precious goods like this. But I myself, haven’t seen it in the ordinary fish section of the store. I would guess that a regular fishmonger shop would be able to find it and order it.)

Recipe Tuna belly cutlets – Yellow Zafferano recipe