I have been trying to get this data for months now. There is not a single site including nseindia which provides the historic implied volatility. Everywhere it is only the stocks hostoric actual volatility. The solution I am using is to manually maintain IV of each stock daily from NSE option chain page. My wife does this for me (for all 170 odd fno stocks) by the time I return from office. Yes she is so sweet;) But I am trying a macro to automatically pull the IV in excel to unburdon her.
Implied volatility changes every second and is different for every strike of option and it is impossible to capture for all strikes so we can consider IV on daily basis and at individual stock level.
Niftyvix index captures the same for nifty so index is taken care and data is readily available.
Coming to stocks there are data vendors like esignal,optionwin,tickerplant who provide these details for a premium, one can contact them or as mentioned above one can start saving them on daily basis.
Implied Volatility(IV) is basically required to calculate the option price. The value of IV is of less significance historically as the volatility averages out over certain period of time.
I believe, the relative volatility is w.r.t volatility of different option strikes in comparison. So to enter BullCallSpread/BearPutSpread strategy, i think current volatility values of different option strike is enough.