As I write this, the S&W website shows almost 20 different variants as being available, and the shelves at my local Academy Sports were well-stocked when I visited last week. Additionally, the staff at this Academy store usually puts all discontinued guns on a special "Clearance" shelf, and the SDs weren't located there.
My guess is that it's a temporary local supply drought.
FWIW if a local dealer has none and is telling you that he can't get any, he may be taking sales lessons from a car dealership on how to convince buyers to pick something from current inventory rather than leaving empty-handed.
If you want one right away and can't find one locally, check out Gunbroker. I just got an SD9 in FDE for $240 shipped, and bought two more brand new 16-round mags for $20 each. There are deals to be had if you search.