Initially, I contacted companies. Now, a lot of times companies contact me. If there is something that I think my readers would especially be interested in though, I'll still contact the company. You can find the marketing or pr contact person's information on most company websites.
I think that it's important to approach focusing on what you can offer the company in exchange for their product.
The only review network that pays you a stipend is Mom Central, but not all the time. Depends on what product you're getting to review.
Signing up with a review network is the best way to get products to review. A lot of publicists and companies are contacting me to do reviews, too. I have a "I do reviews" page up at my blogs, which gives them facts on my blog reader demographics and blog stats, how to contact me, and that I do paid reviews as well.