From what I use by myself:

Open sources (for example, foreign market research): Google via VPN + Incognito mode + advanced search; analytical reports on regions of interest; analysis of competitors (their services, work format, and etc.)

Questions to real people: in communities of practice (facebook groups, Slack), on websites like this; on the forums; through your networking;

Order from specialists: freelancers, agencies that either advise or conduct some kind of research;

Similarity Customer development: for specific tasks, such as researching customer needs

What else can be added and what, in your opinion, is more effective?