Missing sale deed

In 2006 we had purchased a commercial property in Mumbai through SARFAESI Auction from a Bank. After using this property since last 15 years we have decided to sell the property. While we have the original Agreement between us and the seller bank but we realize that the Original Sale Deed of 1981 between the Builder and our Seller is missing. We were only given a photo-copy of the sale deed by the bank from whom we purchased the property. The seller Bank says that since it’s a 15 year old matter they cannot locate original sale deed anymore. What is the procedure to apply for a duplicate of this sale deed from registrar office .