From the Post "Ask Blogging Hints", two of our readers asked, "How do you get items to giveaway from companies?"
From my personal experience, companies came to me. I added a "PR Welcome" banner to my Busy Mom's Tips blog and also a page titled "PR and Advertising" and companies began contacting me asking if I was interested in hosting a giveaway or doing a product review for their company.
Another way to find companies is look at other blogs that are hosting giveaways and see which companies they are hosting the giveaway for. Then you can contact these companies directly via email and let them know that you're interested in hosting a giveaway on your site. Most company representatives are very approachable and will let you know if they have anything available or if they only work with blogs with a certain page rank or level of traffic. In most cases, if they have prize packs available, they allow you run a giveaway for one of their products.
So in a nutshell, if you don't already have a "PR Welcome" banner on your site, add one and make sure you include a way for the company to contact you via email with their campaign.