Similar to SP, SMD tokens cannot be purchased directly on an external exchange. SMD are primarily earned through contributing but can be purchased by converting STEEM tokens to SMD tokens.

Actually Steem Dollars can now purchased on external exchanges !
https://poloniex.com/exchange#btc_sbd
https://bittrex.com/Market/Index?MarketName=BTC-SBD

PS Abbreviation of SBD = Steem Backed Dollars
or just SD = Steem Dollars (not SMD please edit)