Both of these institutes are now combined in the same building. Some early Isotype charts (related to the publication Gesellschaft und Wirtschaft, 1930) were donated to the related EHB (Economic History Library) in 1932 by the Gesellschafts- und Wirtschaftsmuseum, presumably in connection with the establishment of the Mundanaeum branch in the Netherlands. These charts are now kept in the collection of NEHA. The IISH has collections of personal papers relating to many different individuals, some of which contain correspondence from Neurath. The catalogue can be searched on its website.
Haags Gemeentemuseum, The Hague
The Afdeling moderne kunst has a 'Gerd Arntz' collection, mainly consisting of fine art prints and working files incorporating a collection of pictograms designed by Arntz. These files range from early pictograms of the Vienna period to later examples designed by Arntz for the Nederlansche Stichting voor Statistiek, which became the Centraal Bureau voor Statistiek. A representative selection of them have now been scanned and are viewable online at the Gerd Arntz web archive.
Vienna Circle Foundation, Noord-Hollands Archief, Haarlem
This archive contains nothing specifically concerning Isotype; it holds the philosophical and scientific papers of Otto Neurath and Moritz Schlick.