have you tried this site???? not sure if you can just pick them up on here. I am sure Pamps Pete can give you the answer. sure you can get them in the resort but a bit more expensive I would assume!! very jealous by the way.. March is a long way a way... have a good one!!
me brother has just told me that the tour operator still allow for add ons up until the day before departure so i think it seems cheaper to do it through them.