Declaration suit or succession certificate?

My parents died at 1980. We didn't get legal heir certificate as myself and my brother were minors at that time. We have one property in my mother's name. When we try to sell it, we are asked to produce legal heir certificate by the buyer. What's the procedure to get succession certificate from the court and how much charges it would cost? One lawyer says We need to file a declaration suit and another lawyer says We need to apply for a succession certificate at court. Which one is correct? Declaration suit or succession heir certificate ? Please advice. Note: Getting legal heir certificate from taluk office is ruled out as they say they can't provide legal heir certificate for the person died forty years back.