I'm with Barb on this one. I haven't given phone cards in the past, but I will on the next cruise.
For me, the phone card is more of a token of appreciation than another way of giving extra cash. That is, if I'm going to tip extra cash, I do...but then add the phone card as well. If I'm not going to tip extra cash because of problems with service, then I still have the option of adding the phone card or not. I would never give the card in lieu of extra cash.... it would just be an "extra" extra.
As I said in a post in another thread, one of our servers had specifically mentioned getting off the ship in port to call his mother. For this CM, a phone card is great.... something he will use AND it saves him the bother of having to go track one down in a local store.
If a CM isn't likely to use a phone card for whatever reason, they have the option of "re-gifting" it to another CM that may have done them a favor. Just like I might take brownies into work one day.
To answer the original question, you can get these cards at Wal-mart or Sam's. They will be the international ones and come in many different values. Just keep in mind that they value assigned to them is the "domestic" rate and calling another country for the CM will take multiples of minutes... that is, a 60 min domestic card may be worth 20 minutes to another country. I think Sam's bulk packages of cards... 10 cards of 45 mins each or something like that.
hope that helps,
Julie