Hii!
There are a number of diploma courses you can opt for after completing BDS course. Some of them are - fellowship in asthetic dentistry, PGCOI, PGCE, fellowship in cosmetic dentistry etc.,
Some of the college or academies offering such courses are -
* King George's medical university, lucknow
* Aligarh Muslim University
* Dr. D Y Patil dental college and hospital, pune
* YBN university, Ranchi
Etc.,
The link given below may help you with more information.

https://medicine.careers360.com/colleges/list-of-diploma-in-dentistry-colleges-in-india

Hope this helps.
Good luck!