From reading questions on Hubpages and other favorite sites. Also, from reading books. Walking in nature inspires many thoughts and feelings I want to share. Doing keyword research helps when I don't know which idea to start with.
Journaling in private until I get past the muck. When I notice I am finally writing on a topic I have been thinking about, I stop journaling, and go start writing the article.
Finding topics to me, is the easy part.
As others have said here, ideas are all over the place, and often I can't keep up with the multitude that churn off the top of my head. Acting on them is the challenge.