You don't need an example if you care about the job and what the company is doing. Just speak from your heart, and tell them why they should hire you. Why do you want the job? Why do you care about the company? What do you want to achieve there? What experience do you bring that you think will help them with their mission? Show them you aren't just submitting the 351st application for a job. Show them you want to work there, for them, that you're the right person for the job. ...And then bring it down a few notches so it sounds 'professional' (confident without being needy or arrogant).
I don't have any handy. I was just searching the web for a good example and -- hoo boy! -- there are so many terrible examples out there.
All I can offer you is general guidance, then. This is just my opinion, of course, and there are plenty of disagreeing opinions out there. So take that as you will.
To me, a good cover letter conveys four things:
1) That the candidate has at least some knowledge of the company they're applying to and what sorts of problems the company is likely to need solved.
2) Why you want to work for that company in particular. Highlight how what the company does ties into your personal interests.
3) Summarize and contextualize your experience in a way that explains clearly how your experience will benefit the company. This is your chance to call out any special skills that may be particularly relevant to the position, but aren't clearly called out in the resume.
3) Explain anything in your resume that might not look that great. Did you do a lot of job-hopping? Address that. Does it look like the position you're applying for is a bit outside your normal experience? Explain why. That sort of thing.
A cover letter shouldn't reiterate what the resume says. The resume should speak for itself. A cover letter is just that -- a letter from one person to another. It's where you show who you are and what special value you offer to the company.