Look for a topic where: 1. The people who search for your keywords are looking to buy something. 2. The thing they're looking for can be bought online, or it's worth so much money that even a referral is worth a lot to advertisers. When you've thought of a long list of topics like that, see which ones are in high demand and what the competition is like. Pick one that has higher demand and lower competition, and build a site around it. The topics can be anything. People will point you toward real estate, legal services, debt, credit cards, contact lenses, and the like, but some people have made profitable sites about little topics like dog clothes. There are plenty of mid-level topics that aren't swamped with competitors and that have good PPC. You just have to be creative, have some common sense, and spend some time observing the buying habits of the people around you.
I use pulse ebay for niche, it great to see niche there some time i go to google keyword for looking niche keyword hope this useful for you
You don't need to search for niches. Best thing will be to create a niche of your own. Just start blogging about whatever you think yourself perfect in. And then start displaying ads on it. May be you interested in one of the ad-serving networks from my signature.
Look around, what is on the top on people interests? Next, find out competitors activity in this area over the net...