In Indian markets one can use RBI 91 day Treasury bill rate as risk free interest rate You can get the rate from the RBI website, RBI has made it available on their landing page.
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Use reverse calculation. Nseindia shows Implied Volatility values in their website. Using an Option calculator, find out what interest rate is being used in nseindia exchange for arriving at Implied Volatility. There you go. You got an interest rate that is a constant across contracts of varied expiry dates. This helps in comparing if the current contracts are expensive or cheap relative to older contracts. For relative comparison, you don’t want to use a floater.
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