You don't need tickets to get into Discovery Cove, you need to make reservations. Because the reservations are controlled by Discovery Cove you won't be able to find any other sources for tickets. So if they are sold out for the time that you want to go then you are pretty much out of luck.
Because they limit the number of visitors to about 1000 per day, you need to make reservations many months in advance. For a trip at the end of March it probably would have been best to start thinking about making reservations last October.
The only thing that I can suggest is to keep checking with DC. It's possible that enough people will cancel their reservations to free up room for you. But don't count on it.