Where can I find population data that I can link to zip codes for BE or NL?
Note that population for postal and political regions are not often provided in the same public dataset.
An option for Belgium:
2013 population data from the government site - LINK (see right side of page)
Excel file for "population by commune" - LINK
unfortunately, uses "code INS" and not "postal code"
can join population data with postal data from BE post office, on commune name - LINK (see Excel links on right side of page)
- Excel download, sorted by postal code - LINK
(It's just a bunch of Googling. If it's useful, I can do the same for the Netherlands.)
For the Netherlands:
As far as I am aware this data does not exist at zip code level. It does however exist at neighbourhood level. The Dutch bureau for statistics publishes this data in their 'wijk- en buurtkaart': https://www.cbs.nl/nl-nl/dossier/nederland-regionaal/geografische%20data/wijk-en-buurtkaart-2016 (no English translation)
You can also retrieve many of their other datasets through a python API:
The introduction to the data portal can be found here: https://www.cbs.nl/en-gb/our-services/open-data