One of the best sources, for me at any rate, is other hubs. Reading through the work of other writers very often sets of a spark of an idea. I also browse the internet and often an article or book etc. will set off ideas. I've also very often just flicked through the daily newspaper and a few of the articles have given great ideas for developing articles.
In addition I also draw on my experience as a nurse and I also use my knowledge of subjects that I have studied and enjoy to write articles.