Seems to me you either have to get permission from the manufacturer, permission from somebody who took a picture of the product, or take a picture of it yourself.

I think you are covered by fair use if you take your own photo, and you are reviewing the product.

Otherwise, it's not fair use - because somebody else may have a copyright on the *photo*.

Photos of products can often be found on the manufacturer's website, in the "press" section. You may also find blanket permission to use the photos, and/or conditions of use, posted there as well.

Rather than just nabbing images from manufacturers' sites, it would be far better to make contact with the press/media/communications departments of relevant companies and:

1) let them know you're going to be doing product reviews and would like images to accompany them.
2) ask to be added onto their press mailing list(s) so that you can receive press releases announcing future product launches, etc.
3) ask if it's possible to receive samples of "widgets" for testing/review.

Syzygy

Second what Syzygy said. The company PR/marketing people generally fall over themselves to provide images for editorial and review purposes - although they may cease to be helpful in the future if said reviews lean towards the negative. ;)

In some industries, the company website may also have a media area containing photos you can freely use.

you can also try browsing for sites offering royalty free images