Ну вот я нашла это:
You should always check that your tourist card is stamped on the reverse side by the tour operator that issued the tourist card. If it has not been stamped you will have to wait in line at Cuban Immigration for a new one to be issued, which you will have to pay for. The reverse side of the tourist card does say: 'It is invalid without stamp of issuing entity'. Airlines may refuse boarding as well.
здесь:
http://www.tripadvisor.com/Travel-g147270-c104245/Cuba:Caribbean:Tourist.Card.Visa.html
И вот здесь
http://havana.airportcuba.net/customs.html тоже написано, что агенство может выдавать.