I've just done some research here since I'm considering installing fog lights for my 2017.
Med Center Mazda, an online dealer that I bought my M3 floor and cargo mats from, sells the 2017 OEM LED kit (which includes the stalk switch and the lights) for $314 (including shipping): mazdaparts.org/mazda-3-fog-lamp-led.html
The part numbers:
Stalk switch: BALN66122
This seems like a fair deal for the kit, you could save a few bucks by goin elsewhere but Med Center seems like a legit business so I'll stick with them.
I was also looking for high-quality non-OEM options, but can't find any. Some talk about MORIMOTO's LED and people seemed to like them, but they are apparently no longer available, see last pages: mazda3revolution.com/forums/2014-2016-mazda-3-skyactiv-how-guides/104482-installing-morimoto-mazda-type-z-led-fog-lights-5.html