Aer Lingus sell them on board on the way over (five euro or ten euro ones if mmemory serves me well). We bought a ten euro one and made about seven calls to home. Still credit left I think.
There are cards you can get in convenience stores in New York that give you over an hour for $10. Loads of different brands - just ask the guys in the shops. Cost per minute is next to nothing - main cost is the connection fee. Therefore not much good if you make lots of short calls but great for the long weepy calls home to the family.
Buy time out when you arrive - they have listings for everything that's going on on the city from museums to gigs. Includes some tourist stuff also.
My sister travels to the US a few times a year and always buys one of THESE cards. She has tried a few different kinds but finds that company is the best