There are only 3 exchanges on which SBDs can be traded. The exchange with highest market cap of SBD is Bittrex (i use this one), on second number comes Poloniex (but it has some security flaws and few people lost their funds.. third one is HitBTC, but they are not accepting SBDs right now ..
So come to think of it, you really have one option and that is bittrex.

U can also sell your SBDs in internal market of steemit and can get Steem for it.

There is another way. You can use blocktrades.us to exchange your SBDs to other cryptos (limited options available) and those exchnaged crypto funds can directly come to your offline wallet or even at exchanges.

Let me know if you still have trouble understanding something in this process..

Best wishes :)