Want to find details of the ex-husband of my girfriend

your girlfirend can provide this. why to do this backdoor activity?

Perhaps you could do some digging on social media. No law against stalking a guy. :) 
Happy Hunting.

Otherwise if you do know certain details as to the previous place of marriage or something, you could verify it from some Marriage Registrar, but again, Social media would be more convenient if the content is available.

Its Vague to find at backdoor method. if  you still need it, go to Concerned court, find a experienced Advocate's  Clerk. explain the facts and make him to search for the detail in Suit/O.P Register for the date. but it takes more time and you have to spend more.

Consult a lawyer at Banglore with paerticulars. You can also search on court website.