There are a wide range of exchanges that allow you to buy, sell and trade NEO and GAS. Please be aware that not all exchanges will allow you to claim GAS, and that NEO is not divisible. So even though you might be able to trade NEO in fractions, you will need to withdraw in whole numbers (plus fee).
In general, it's not recommended you use exchanges as a place to store your NEO or GAS long term. For storage options, visit our wallets page. Also, always double check the URL of any site you visit via a link to ensure it is authentic, legitimate and the site you intended to visit. Even when being linked from us!