I don't think RAM or Chevy even make an extended cab anymore, only Ford. I searched the whole state (CA) and found 2 on the lot two years ago when I bought mine. One in Corning, one in Fortuna. So no, they don't order them.
From Ford, last time I looked you could still get an extended cab in any trim level. My 2015 is a Lariat. Maybe you can't get a Platinum? A crew cab is heavier and floppier and has a larger turn radius (if the steering gear is the same). From Ford's site it looks like you can get a 2018 F250 or 350 but not a 450 pickup in extended cab, up to Lariat. You can get a 350, 450, or 550 chassis cab in extended up to Lariat. But you will probably have to order it.
