You won't be able to rent a fixie since nobody rides them over here except for a few messengers and hipsters such as myself. But you will find plenty of bikeshops that can sell you a single speed beater-slash-workhorse for less than 50 euro's, don't buy them from junks. Also, in bigger cities such as Den Haag or Leiden, don't park pretty bikes out on the street unless it's with a few hefty locks attached.