Bittrex
https://bittrex.com/Market/Index?MarketName=BTC-SDC

You can see what exchanges coins are on by using coinmarketcap.
e.g.
http://coinmarketcap.com/currencies/shadowcoin/#markets
I trust bittrex over the other two.

Mintpal v2 will have it on the 17th, 6PM GMT
http://blog.moolah.io/2014/09/01/mintpal-v2-tentative-market-listing/
https://twitter.com/MintPalExchange

Cryptsy will add SDC themselves and soon if they know what's good for them.