Where can I find a sample business plan for starting my own talent agency specializing in booking tribute artists and celebrity impersonators?

There are many samples and low cost versions online. Here is one (http://www.thefinanceresource.com/free_business_plans/free_talent_agency_business_plan.aspx) that is pretty comprehensive.

I also suggest you look into using SCORE (http://www.score.org/) which is a free business resource funded in part by the SBA. Retired executives and business owners act as a Business Mentor, and it's free!