There will be times that you have an interest in a particular company but are unaware of a specific job opening. You should always try to first network your way to an introduction into the company. But, if those efforts are unsuccessful, try contacting the company anyhow, using a cold contact cover letter written to introduce yourself and show specifically how you could add value to that company.
This cold contact cover letter is an example of how you would do that. It is worth your time to do some research on the company and determine exactly what challenges and problems they are facing for which you might be the solution. Your cover letter should be very employer-centric and illustrate that you understand what their most pressing needs are and that you have the proven ability to meet them. Most importantly, follow up the mailing of your resume and cover letter with a phone call. The follow up is key when you are using this job search technique!