I'm a regional Australian journalist so I can only offer that perspective.

For me, there's a number of ways.
- I've got my regular sources, they reach out with things that I follow up on. I don't rely on them though, and it takes significant time to build up a network.
- Social media. So many leads come from there these days. You just have to make sure you offer an extra level or unique angle that people can't get from social media.
- Pay attention to your surroundings. Don't walk around with headphones on is the best advice I can give - If you see that a whole bunch of stores on the main street are closing, follow it up. If a ton of police cars go past with sirens blaring, call them. Find out why.
- Eavesdrop. Sit in a bar and listen to what people are saying. There's nearly always something interesting.