Alice Davies's reply:
Well done for acknowledging and understanding that the content, style and voice of your CV and covering letter is going to be key to getting your application noticed. Your application is your first step to getting in the door.
I would suggest that you contact some small independent design agencies that hire product designers (try your local ones first) and ask if they would be able to share with you what they would consider an awesome application. Be open and honest and say that you are looking to move into product design and would love to see what experience others have and how they have approached their applications. You never know, one of them just may be interested in hiring you as an intern! Use LinkedIn to make some contacts and ask around on Facebook to see if anyone knows anyone who hires product designers and would be willing to have a 15-minute chat with you.
I know that there are agencies and coaches who do focus on CV and cover letter help but I don't know of any specifically for product design. The lovely Debbie who owns http://synergycareermanagement.com/about/ would be able to help but she isn't specifically focused on the design world, but I know she helps people with CVs, letters and interview techniques.
Best of luck.