If you don't know any relevant news sites for your niche have you considered you might be in the wrong niche?
Most news is circular, so the first location is going to be your competitors and sites in similar but not identical niches. Just take a look at any newspaper site you will often find "first reported in xxxx" style comments in articles before they spin their own take.
The second location would be the larger news outlets, Guardian, NYT etc
Third directly to source Reuters, Associated Press, Press release sites
Once your site has grown to a point where you are considered an authority you can always ask for direct submissions, paying for exclusives etc.
Ultimately if you reach the point where you are hiring journalists they will have their own contacts and ways to gather stories.
For Photos their are several specialist photo journalism out there, for example Getty and general stock image sites.
With the rise of social media it's not hard to find news in virtually any niche the key is of course verifying the story.