It sounds like you are already very familiar with the piano industry.
I’m somewhat of a novice regarding pianos but as a matter of courtesy to you as a new member here I will offer my opinion. (By-the-way, welcome to the forum.)
As VGrantano said, the competition for used S&S pianos in unrestored condition is probably stiff; so, chances are you will not be able to find a single source for this type of pre-owned pianos. Also, I would imagine that other piano dealers will be reluctant to reveal their sources to a possible competitor.
Therefore, if I were you I would begin an advertising campaign in various piano related publications, websites (Craigslist, etc…) newspaper want ads and so forth stating that you are in the market to buy pre-owned S&S grand pianos in unrestored condition. Also, you can begin an extensive internet search for various classified advertisements offering used/pre-owned S&S grand pianos for sale.
I hope this helps (but I’m sure you knew this already