Unfortunately I am not aware of another source for historical data regarding the supply. Coinmarketcap is notorious for not displaying accurate data. For example, right now it shows a circulating supply for steem of 345 million but the actual number is 326 million. @penguinpablo makes a daily and a weekly post with alot of stats for steem (including the available supply).
Selling sbd is not the same as converting it to steem. It is important not to confuse them. One does not create steem and the other one does. The first one is an exchange between two parties on the open or the internal market, the second one is a special blockchain operation carried out by one account (no selling involved).
Converting SBD will only create an amount of steem above the inflation schedule if the steem price is significantly different between the creation and convertion of SBD. This was the case at the end of last year and the begging of this one. By my estimation we have an excess of SBD in the amount 2.6 million which, if converted would equal an increase of about 7.9 million in the steem supply (or about 2.2% more inflation than what is scheduled). Now if the steem price goes to 0.21 USD then we don't have an excess of sbd and there is less pressure to make conversions.
Just to clarify, by excess I mean the amount of SBD that is not supported by the virtual supply to maintain the peg to the US dollar.
One last point, converting SBD to steem does not necessarily mean that there is higher selling preasure for steem. If the steem obtained from the conversion is not sold on the open market then it has no effect on the price.