Poloniex or Bittrex are the bigger two that trade SBD. Both are having problems with their ID verification however, so getting into them seems to be a challenge right now.

I believe Bittrex stopped accepting new traders. I have been waiting going on 4 weeks to get verified there.

Poloniex is running behind on their verification process as well.

The only one I've been able to get into the last month has been www.binance.com/?ref=12587112 but they do not trade SBD