I would get it there so you don't have to haul that there, taking up luggage space and weight. We bought some at the Target in Kapolei after arriving. But if you plan on snorkeling at Rainbow Reef, I believe they allow you to use it for the whole day so you can take it to the lagoon too. You can buy a day pass for $20 or a length of stay pass for $39 - I believe these were the prices, or pretty close.