go to opensourcecms.com and there is a boatload of open source cms systems. Choose the one you like. With some work you can modify any of them to fit your needs completely.
go to sourceforge.net and search cms and u can also select ur catagory to search. there's alot of cms and u can get ur one
Despite what anyone says, wordpress is not only a blogging platform and is usually the best choice for a cms too. It is extremely easy to use, has thousands if not hundreds of thousands of plugins for it. Also, It is easily customizable; not just the theme, but the framework as well. Also, it is updated frequently so it is very secure. http://wordpress.org/download/
wordpress is my recommendation too, I use it as a cms on a host of my sites.. It's very easy to configure and theres a lot of themes which do what you need out of the box.