See if there are other people selling products in your niche already. For information products look at affiliate marketplaces such as Clickbank and JVZoo and see what products have the greatest sales volume.
For other types of products go to Google. Enter your niche name + affiliate program . This will bring up any business that runs an affiliate program in your niche. This is another way to get ideas that you haven’t yet thought about.
This is also a great way to get started in a new business. You don’t need to worry about product creation nor customer fulfillment and service. Also the vender will most likely be able to supply marketing tools such as e-mail swipes, banner ads, images, keyword lists and results of EPC campaigns they have run directly themselves. They may also know the demographics of their target market really well.
For physical products you can look at Amazon and see if you want to set up your own Amazon shop.