I looking to build a niche CNET type of site. My niche is reviewing fitness products like FitBit. How can I get companies to provide me a review unit? I would return it back after testing it out and reviewing it for them.
Contact someone in their PR department with your intentions. If you're targeting a niche that they would like to break into then you'll have an easier time. Otherwise you might have to purchase fitness products at first until you build a more significant audience.
I used to write reviews for video games. It was notoriously difficult for new sites to get review copies of games let alone actual hardware. However, since I was one of the first sites to serve women gamers it was much easier to get the attention of big companies like Nintendo.
This is a bit chicken and egg. If you have an empty site and no visitors, it's going to be hard to justify asking for test units.
As Garet Claborn says - you should just ask them.
If they say no, then you either buy a unit of your own, or borrow one from a friend, or find a stranger with one, and give them something small in exchange for borrowing. Once you'd got a bunch of products reviewed and traffic to your site, you'll find it easier to get test units.
Have you tried simply contacting them? Many companies are willing to send products, without returns, to reviewers. Direct mail / cold calling a corporate office is rather hit/miss - so try to find the best positioned person within the companies to contact.
Aside from this, especially to get started, you can just go to a few gyms. Write down the model of different equipment you use and compare different ellipticals, resistance training sets, etc..