Originally Posted by guv1976

Unfortunately, Premium Economy is only offered on a relatively small number of domestic routes. It's far more common on inter-continental routes.

I believe that on domestic routes, both AA and UA do offer fares that include seating in the extra-leg-room section of the Economy cabin. If you obtain status with AA or with some of its oneworld partners, you can choose Main Cabin Extra seats for free in advance, even on Basic Economy fares. (How far in advance will depend on status tier.)