Lots of sources, mostly science books, and websites. I used to have a subscription to Nature magazine (IMO, the best magazine for general science news around). I get news from Google, and NPR--I'm not really big on keeping up with current events.
When I was younger, I would make-up math and physics problems for my self to solve, and I'd be thinking about them while I went about my chores--didn't always work out well for my chores. Still, a great amount of my "knowledge" comes from my own thinking about subjects.
I also go to libraries a lot. Luckily, there are some real good university libraries around, and I can read about almost anything.
As much as I hate to say it... school has helped me become more knowledgeable. I don't like homework or the "drill-things-into-your-head" format. But I have found that I actually did learn quite a bit in school, and I've been blessed with having among the best education programs available for the things I wanted to focus on.
I also learned a lot from my job. I used to learn a lot more. I think working in an environment where you have to do a lot of things, and almost everything is under your own control (rather than having all sorts of bureaucratic procedures to follow, and a narrow little role to play) is a good place to learn on the job.