Where can we buy the the hk 220 Express Travel Pass once we arrive in the terminal?

We'll be staying in chai wan for three days. We will be coming from macau so we'll just be taking the ferry to hongkong. Our flight back home is in HK but i figured that it will be more economical if we buy the 3 days unlimited pass with a single airport trip. Can we use the 3 days unlimited ride first before we avail of the airport trip?