Hello Steemian here's a list of exchanges where you can trade steem and steem dollar. Though Users can purchase the STEEM or Steem Dollar tokens directly through their Steemit wallet using Bitcoin, Ether or BitShares tokens.
The exchange I have listed below also have the tokens available and the exchange markets are as follow
Bithumb
BitShares
Bittrex
BlockTrades
GOBADA
GOPAX
HitBTC
Huobi
Poloniex
Shapeshift.io
UpBit and Yensesa.