A lot of avid anglers have become aware of Yellowfin Tuna because of shows like Wicked Tuna - where very limited seasons allow a select few (very) rugged people to fish for them and catch them relatively consistently. There are also some wicked captains that will do it for you too. Here in Central Florida finding tuna is most likely off the costs around Destin, and there we would recommend a guy named Gary Jarvis. He runs a killer operation, is very respected by people that both know him in the fishing politics arena as well as personally. He can put you on the fish.
If you were to fish the east coast of Florida, we recommend Top Shot Fishing Charters.
For some great information about tuna and fishing for tuna, check out this Q&A from the show Wicked Tuna.