These browsers have many advantages, but the main one is the "tabbed browsing" facility. More details
The first place to start is Google. That's because it's the most widely used. You can start your search with general terms like "containers" or "used containers" and then narrow it by using more specific keywords, like “used shipping container”. General keywords produce general results. That's the case with "containers" for example, because "containers" can represent a lot of things. It's good to have an eye on the results too. Use the tabbed browsing.
In this way you can keep all your findings in one window and then you can bookmark them as a group of tabs. If you're using Internet Explorer, open the results in new windows and then bookmark them one by one.
For more specific searches try these keywords:
You can add to the searches the name of the area you live in. For example if you live in Florida you can search for "florida used steel container". Also take a look at the sponsored links on the right of the results page. On many occasions you'll see listed there companies you might be interested in.
For more Google searching tips, see here.