I think siacoin is the best altcoin under 1$ in which you can invest. The concept of decentralized cloud storage is a project which can become popular in future and hence you can expect good returns from it.
Do you know where can I buy this coin? which site?
You can buy siacoin at bittrex for example. But before you do your investment, you should do your own investigations, so you know, in what kind of company you are investing your money. Read their whitepaper, join the community so you can see, whether they are in a stable environment.