all of the US based acura online parts stores run off the same US Dealership network. Each site is actually tied to a different US Acura or Honda dealership, so when you order from them essentially what you are doing is ordering from that dealership's parts department, they are ordering it from the US supply chain or from Honda directly in Japan, the dealership will receive it and then ship it to you.
sites like jp-carparts, amayama, mita, nsx-parts, sos, and a.s. motorsports will actually order it directly from Honda Japan, receive it, and then ship it to you if they don't currently have it in stock. These sites are not dealerships they are the middleman parts supplier (much the same way the dealership acts as the middleman when you order from a US based site).
The way I have come to understand it, the big difference between the international suppliers is they order directly from honda japan without a region specified, so if a part is showing as discontinued in the US Dealership network a US Dealership will be blocked from ordering the part, but the international guys can sometimes still get those parts. Your mileage may vary though.
also note that currently most parts suppliers aren't accepting orders because Honda announced a price increase on parts starting April 1, 2021. So if you ordered from Amayama last night there is a chance they will cancel your order until they are able to update their pricing database. If that happens don't fret, just wait 2-3 days and order again, but don't be surprised if the price has gone up. We are all collectively holding our breath right now hoping prices don't rise too drastically.
