Thank you all. In short, it's a pain. The only ways to buy these supplements are with a dutch bank account, with Euro coins, or at the ticket office in person (1980-style). As someone indicated, the chipcard can only be bought online with a dutch bank account, and the ticket machines only take dutch credit cards. Even the -- otherwise ridiculously expensive -- rail europe cannot sell these supplements and I would have paid up to $10 for the 3 euro supplement to save the hassle.

It's time for NS to come out of the stone age and stop being hostile to foreigners. Someone should write to Timo Huges (their CEO) and explain that NS is not fulfilling its social mission by adding so much red tape to non-dutch residents.

Until then, I will be on the Thalys.