I have searched for many websites such as (http://www.crystallography.net/cod/) and (http://rruff.geo.arizona.edu/AMS/amcsd.php). However, I could not find any structure I want.
you can modify the Zn2SiO4 CIF file by introducing the Co ions in the site you find appropriate. Take a look in the Vesta manual (also in youtube) for further infomation on how to add an ion (or build a new structure) and its occupancy.