Stam got a letter of approval for his project from the European Central Bank and he even cinched the support of the Spijkenisse city council. They financed his project with 1.3 million dollars which was around a quarter more expensive than what a more simple set of bridges would have cost.
It's implied in this article (in Dutch) that the municipality of Spijkenisse funded it. At no more cost than building 'normal' bridges, as they had planned originally
has some other interesting stuff, looks like he's pretty well known so securing funding must not have been a problem - wonder who funded it though.