A great source to look that may help you at is the SBA.gov site. There may not specifically be a sample plan for a clothing line, but the samples are really only to get your mind moving and then you have to sit down and write your own by working out all the questions the biz plan forces you to examine and resolve. It's not like a template website, where you grab a generic design and simply change the title and and some simple text and whalah! it's done. Writing a business plan is a very in depth look at all aspects of YOUR business. Maybe these couple links will be helpful.
Small Business Administration - Get Ready
Small Business Administration - Write a Business Plan
Oh yeah, and these are free
Small Business Administration - Get Ready
Small Business Administration - Write a Business Plan
Oh yeah, and these are free