You can buy SBD on the internal market or a cryptocurrency exchange or Blocktrades. But please note that I was mistaken about when the conversion mechanism guarantees getting 1 USD worth of STEEM for each SBD burned. Getting that amount stops being guaranteed when the debt ratio exceeds 10% and not 20%. Because the spot prices of SBD and STEEM are already what they are, the 3.5 day average of the debt ratio is certain to fall to 10% in two or three days even if the price ratio does not change. I started converting my SBD a few hours ago and plan to trade any SBD rewards coming to me to STEEM right away without waiting for the conversion. Unless the price of STEEM makes a radical turn upwards, I consider it likely that the price of SBD will collapse in a matter of days.