Ticket availability will vary with the opponent. The Yankees-Red Sox match ups always sell out. Try to buy your tickets in advance of the game so that you know that you'll have a seat.
You could also watch for those road games where the Yankees play the Phillies or Blue Jays. Both the Phils and Jays play a short distance (20 miles) from Tampa.
Mid to late March is probably the best time to catch a game. March weather can be fickle, cold one day and much warmer the next. Things begin to stabilize toward the end of the month.
Tickets to games at Legends tend to sell out quickly... but the resale market is decent, except for games against the Red Sox you can often (but not always) find seats near face value.
I have season tickets at Legends for spring training, because it can also be frustrating at times to get the games you want. Plan early.