I don’t think there is an ‘official’ source for these data.
You could get the data from OpenStreetMap, although it might be incomplete or not accurate.
The easiest way is probably to use QGIS. Install the QuickOSM plugin. Create a query. Filter on key = leisure, Value = park and a specific area, for instance Rotterdam. After you execute the query, the data is added to your QGIS project. You can now easily export it, for instance as a GeoJSON file.
Alternatively you could also use Overpass Turbo to query OSM using the Overpass API: https://overpass-turbo.eu/