Determine the reason you are seeking out a co-author. You will save time searching for a co-author if you can advertise specifically what you want them to contribute to the project. A good co-author is one who complements your writing style and brings additional knowledge to the writing topics, and by creating guidelines of what you want in a co-author, you'll quickly eliminate the writers that are insufficient to work on the material.
Inquire with close friends and relatives. The best co-author is someone familiar and trustworthy. If someone in your inner social circle is a writer or an expert with inside information on a topic you wish to explore in your writing project, proposition them about working with you. Though creating a business relationship with a personal relation can strain a friendship, outlining work detail and responsibilities beforehand reduces the possibility of conflict.
Search through writing groups, writing courses, and social networks. Join local writing groups and attend on a regular basis to establish a rapport with local writers. Attend writing courses at local universities or contact professors for writing contacts. Use social networks like Facebook, Myspace, Twitter, or deviantART to advertise that you are seeking a co-author. Websites like StoryMash allow writers to collaborate on fiction projects, self-publish on the StoryMash site, and earn a portion of advertisement revenue.
Advertise in libraries and universities and post ads in online forums. Print out a few flyers, with brief synopsis of the writing project and the kind of co-author you are seeking. Include your first name, as well as phone number or email address, then post the flyer in local libraries or universities. You can also create ads to post online in various writing communities, like Absolute Write or Writer's Digest. If you're writing in a specific niche, like horror fiction or journalistic non-fiction, finding the appropriate online forums for those areas can narrow your search and yield more appropriate writers for your project.