This is not an original article by me but I found it highly useful:
https://www.listenmoneymatters.com/passive-income-ideas/
There are some other sites which are more specifically focused:
eBooks
https://bullshit.ist/how-i-make-400-a-month-in-passive-income-by-self-publishing-68fa948edff5
https://www.thebalancesmb.com/make-money-selling-ebooks-online-4122181
