Johncn two options come to mind given the relatively long time you will be on the island, the sporadic nature of the diving and your location is A. just pick up tanks near where you will be: some at Gowest, some at Lagun Bahia, some at other locations with dive ops such as Bdiving at Cas Aboao. Another option might be a dive card at Lagun Bahia divers for most of your tanks. You can buy a card for x number of tanks and use it as you need it with no time limitation. See Diving - Bahia Apartments & Diving, Look under the tab for pricelist. I only used them once but I liked them. The tank card may give you the flexibility you are looking for.