For cif files, the easiest way is to check the Open Crystallography Database. I found several pentacene polymorphs there. http://www.crystallography.net/cod/4109833.html
I also found .cif files in the CCDC data base and in articles in Acta Cryst C, but I presumed that you didn't have access to those for free. I find that a Google or a Google Scholar search can find quite a lot.