I met with a tuna expert a while back and asked where he got his fish, fully expecting him to direct me to a market vendor or a shop in town. Instead he said he got his delivered. Your mileage may vary, but I've had good experiences with Gene's Premium Seafood (email@example.com email below) as well as Farmright Fish firstname.lastname@example.org
I use the kokoda cuts as I would boneless skinless chicken. The 500 gram packs defrost quickly and cook up easily, these are on my weekly shopping list
Sailfish can be gamey but does well brined or marinaded, can be meaty enough to put on skewers and bbq'd
Blue marlin is sometimes used in sashimi as a replacement for tuna
Is this helpful? What have your experiences been?