In case you want to buy some alt coin that is not available on Indian exchanges, you can check which exchanges support that coin on coinmarketcap.com . Once you know the name of the exchanges, you need to create an account on one of these exchanges. Binance, Kucoin, Huobi, Cryptopia are names of few of the major exchanges that support most of the new and upcoming alt coins.

Once you have an account on a major exchange that supports the alt coin, you can convert your INR into ETH / BTC (preferably ETH as it is faster and cheaper) on one of the Indian exchanges and then transfer your ETH to the major exchange. Then you can convert your ETH to the alt coin which you want to buy.