I would like to order rear JDM headrest, anyone knows a reputable site? They look like this: Japan used Toyota Prius DAA-ZVW50 Hatchback 2016 for Sale-3560434
That's what is in my car - are yours different? Are you talking about a Gen 3 or a Gen 4? The link you sent was Gen 4.
The larger head restraints provide more protection from head and neck injuries for rear-seat occupants. I assume everyone already knows that they fold down, as shown in the Owner’s Manual (PDF), page 303. I’d suggest IMPEX JAPAN, about which I’ve written before, but it may be difficult to arrange cost-effective shipping from Japan right now: Japan Post EMS service to the U.S. has been suspended since April 24, due to the limited number of international flights. Before placing an order with any overseas seller, I’d suggest contacting them to confirm their current shipping options. Japan Post is still accepting surface parcels to the U.S., but this service is slow and not tracked. Carriers such as FedEx and DHL are also operating, but their rates are much higher.
I'd also be checking that they are legal in USA. I know there have been Japanese Spec cars in the past which weren't allowed into Australia without significant safety modifications to meet Australian Design Standards.
I fold my 2/3 of my back seat down when not in use - visibility better (child restraint in other 1/3). That way, I've still got a head restraint if I have a passenger there.