As showed on Coinmarketcap, you can buy smarts on HitBTC, Coinexchance and Stock.Exchange. Cryptopia, which usually has the biggest market cap by far paused all trades. There’s other places you can trade too, like on Crypto-Bridge (see the official SmartCash twitter for more details https://twitter.com/scashofficial), but they are not listed on Coinmarketcap.
Be careful though! For the time being, no exchange will allow you to transfer your Smarts back to your private wallet. Exchanges have to update their wallets before they enable transfers again. This is one of the reasons you see such a disparity in prices (the other and main reason is that Cryptopia’s 0.3$ do not refer to the actual price, but to the last 09/01/2018 trade).
As reported on the official SmartCash Discord (invite link: https://discord.gg/BDUh8jr), Cryptopia should resume trades and transfers tomorrow or early in the coming week.
You probably know i, it is not advised to follow advices from strangers on the web when investing in Cryptos, but if I were you I’d wait until the exchanges update their wallet before buying Smarts. Chances are the prices will drop down.