Few people seem to be aware that the locations of all windmills are available in the official Dutch Topography Registration (BRT: Basisregistratie Topografie), specifically the Top10NL dataset within the BRT. They are available as building (“gebouw”) polygons with attribute Typegebouw like ‘%molen%’. It can be quite cumbersome to find and extract these from BRT/Top10NL though. Alternatively, this extract (and many others) is available from www.spotinfo.nl, with a paid subscription.