This is dependent on the exchange in question, as some exchanges will make a variety of buying options available to the users. But some exchanges can be reluctant to enable customers to exchange altcoins for other cryptos, owing to the relatively small amounts of liquidity in the market. When this is permitted, it is almost always the case that Bitcoin will be the approved altcoin.
What we can say confidently is that shopping around with exchanges and cryptocurrencies can yield better results. The more options that you give yourself, the more likely it is that your particular needs will be met.