Both ANA and JAL allow you to book these directly. ANA is a bit more difficult as you have to select the correct ticket type/fare under Economy and look for "Premium" in the ticket description. On JAL, there is a drop down to select "Premium Economy".

Premium economy is not worth it at the standard price. However, as you mentioned Tommy it will give you an nicer experience from standard economy as you saw the seats. But there are frequent sales and promotional offers where Premium Economy is discounted to somewhere about 1/3 to 1/2 higher than standard economy. Examples, SFO-NRT for two weeks with a mid-week late September departure is about $950 on JAL (ANA is about the same), and premium economy is about $1500.

IMO that $550 premium is worth it when you add it up:

20 hours of 38" pitch and a wider seat (19" vs. 17"), Business Class lounge access, a shell seat where the person in front of me CANNOT recline into my seat table (on JAL not ANA), a wide armrest that BOTH people can use and dedicated check-in. (Premium Economy in NRT or Business Class check-in ex-NRT), foot rests and a power port.

What really bites is that you cannot redeem American Airlines AAdvantage miles for a premium economy ticket on JAL, even though it is a One World member. The award just doesn't exist to AA.