They were on the bar menu when Joe's opened, later dropped, but you can still special order one. At least you could when I asked last year. Good luck. This is not something that is often very good outside its native habitat (the Gulf Coast of FL), and is most often not made with actual grouper, even in FL.
For my money, the walleye sandwich at Culver's is easily the most true local version of this sort of thing we have. They use a nice, big, beer-battered filet, not a breadcrumbed hockey puck like most fast food fishwhiches. Tell them to make it with burger fixings and you might as well be at Frenchy's in Clearwater.