pick up a card and call them! The online one I have used in the past is speedy pin http://speedypin.com/phone/cards/Jamaica they have a selection to choose from and you can buy in 5 dollar increments. They are reliable--I know as I spoke to my ex weekly for a couple of years using it.

But as likkle red said, you can also pick up a card at most drugstores or gas stations.