Try NissanPartsDeal, Amayama, and Partsouq. Otherwise you're stuck with with aftermarket. I got OE head gaskets from Amayama, shipped from the UAE, somehow cheaper than NPD had them in (I assume?) the US, so check them against each other. But sometimes Amayama turns out not to have what they think they do. I tried ordering a dipstick tube from them, only to get an email the next day that they didn't actually have it. I think they take the order and then see if they can track stuff down from their suppliers.
I haven't ordered from Amayama or Partsouq since the tariffs got weird, not sure how they're dealing with that. My dad tried ordering parts from the UK recently and was told they had no idea what the charge would be by the time the stuff got here, and they just didn't want to deal with it, so they weren't even accepting US orders.
I got the OE tstat and water pump ("pump ass wat" in Nissan parts speak) via NPD, NSK idler and Bando belt off Rockauto because I couldn't find OE. Can't speak to the quality of either, haven't even installed them, given the engine they were going on turned out to be a doorstop. Years back I put a Gates belt and pump on my VG30, and they're still holding up 30k later, but I've heard some complaints about Gates since.