Have you even tried ordering one from S&W?
I have enough of the mags, but I still make periodic orders for replacement mag springs (same mag spring for the 8-rd 3913/908 series, and the 4040PD).
Don't be surprised if they're back-ordered, as I've been told the company has been experiencing a LOT of orders for all their various mags (including the 3rd gen's discontinued for commercial sales).