One way or another your bank has an IBAN you can use, but if it's a smaller bank, they may go through one of the larger ones. You just need to find the right person to talk to at the bank.
Even if US accounts had an IBAN (they don't, US banks don't participate in that standard) that wouldn't help. The Dutch bank can either process payments via SEPA (Single Euro Payments Area) or international wire transfer (which doesn't need IBAN but is expensive).
Maybe GP can make them use Transferwise, Currencyfair or the like?
