Dear Amit, 

There is no seperate website for this. The official JEE website itself will include this option for uploading OBC certificate in the period of time that they schedule. Hence the link for official website is :

https://www.google.com/url?sa=t&source=web&rct=j&url=https://jeeadv.ac.in/&ved=2ahUKEwjeu8ST0r7jAhWFtI8KHciCC10QFjAAegQIBxAD&usg=AOvVaw3Pcn8sn3B66W5UCXwS-KPV&cshid=1563458996574

Hope this helps. 

All the best.