Most of the Cactus League games we went to were sold out or very close to it a few years ago. I've had luck just buying tickets at the window for Grapefruit League games.
Situations like that. It really does depend on who they are playing. Some days you can walk up and buy front row.
Last year we went to a game vs the Phillies that was sold out(I think because of Harper). We had purchased tickets well in advance. I dont remember seeing too many available on the street the day of the game. I would suggest having something in hand before you go.
What game are you going to? Ill be there 3/10-3/15. We are going to Cards @ Mets up in Port St Lucie and a game at Roger Dean(I cant remember which day).
Enjoy....I love it down there in March.