If there is one thing the internet is good for, then it has to be research. And that applies as much to the work you are doing as to new sources of work. At the very least, the web can give you access to up-to-date company information such as addresses, phone numbers, emails, personnel and organisational structures. Use it to find out more about companies you think you could work for.
Check their press pages to see if they employ a PR agency that might be interested in your writing skills. Do they have a customer magazine you could contribute to? Is it worth talking to the marketing director about the content of the website itself? Does their job application page mention whether they need freelance writing help?
Like any form of research, however, trawling the internet can be very time-consuming unless you have a good idea of what you are looking for. I would advise using web research as part of a wider, structured plan for finding new business.
You might decide, for example, to approach all the direct mail companies listed in your local Yellow Pages, in which case checking their websites would be a good way to get extra information about their businesses.