Corelan Cybersecurity Research[1] is a good starting point. There haven't been any new articles in ~5 years, but some of the techniques in the linked article are covered. The /r/netsec subreddit is also a suprisingly good source of security info. A big part as well is just experimenting on your own. Vulnerability research is a very "creative" field, so being able to think outside the box is just as important as having technical knowledge.
[1] https://www.corelan.be/