Google site-searching is the best way to discover primary sources from European government websites. You first need to know the top-level domain of the country. You can use the Wikipedia site to discover this: https://en.wikipedia.org/wiki/Country_code_top-level_domain.
For example, the TLD for Spain is "es", Austria is "at", and Greece is "gr".
Search Google Web first to discover what the government subdomain.
For example, you might search: site:es government. We see that the government subdomain is: gob.es.
It appears that Austria uses .gv. So the subdomain is: gv.at.
Greece appears to use gov, thus: gov.gr
From there we can now discover primary sources from in-country government websites. Examples:
site:gov.gr citizenship OR site:gov.gr citizenship filetype:pdf
First, find out "who cares" about your topic among international organization. Some examples of relevant IOs might be:
European Union: europa.eu
European Commission: commission.europa.eu
United Nations: un.org
International Organization for Migration: iom.org
Next. Do Google site searching to find specific documents:
site:europa.eu migration hungary
site:un.org citiizenship belgium filetype:pdf