There are several major siacoin exchange you can buy Siacoin.
Based and fully regulated in the USA, Bittrex is the go-to spot for traders who demand lightning fast trade execution, stable wallets, and industry-best security practices.
ShapeShift is probably the fastest way to exchange cryptocurrencies and blockchain assets there is. Using this service, users can exchange cryptocurrencies in a matter of seconds. ShapeShift greatly reduces the risk of stolen funds and it increases privacy because it requires no account or personal data to use.
Poloniex is a US-based cryptocurrency exchange offering maximum security and advanced trading features. Start trading in minutes. All it takes is a few simple steps.
Bitsquare.io has an open source peer to peer system that allows anyone to sell Bitcoin in exchange for international currency (FIAT) like US Dollars, Euros or Yen, and alternative cryptocurrencies. Bitsquare proudly announced its Beta release on the 27th of April 2016.
YUNBI is a future-oriented exchange for blockchain assets. Founded in Beijing, 2013, and built with the code released by Peatio Open Source Project, YUNBI adheres to the principles of integrity, transparency and being open-sourced, offering customers a safe and easy trading platform.
Note: It is fairly easy to buy Siacoin on Bittrex, Shapeshift, Poloniex, Bitsquare, and Yunbi. Simply follow their instructions for setting up an account and depositing funds. As these cryptocurrency exchanges offer a different environment for acquiring Siacoin in that you must buy it by fulfilling other traders’ sell orders. This means that prices may vary, sometimes widely.