Dry sump is required in some applications but for 99.9% of us, the wet sump works a treat.
Check with any of the replacement parts suppliers & I'm sure that you will be able to source a pump through them without difficulty. Repco & Supersleep come to mind but there are many others.
The pumps were supplied to Datsun/Nissan by vendors, or outside manufacturers & they didn't stop providing replacements just because Nissan decided to discontinue servicing this part.

Option 2 is to make up a high volume pump by utilising the rotating parts from an L series pump & an A series pump.
The conversion is fairly straightforward but it must be remembered that the hole in the pump body is 12mm while the L series shaft is 1/2" [12.7mm] In my view, the best solution is a custom made stepped shaft that has no slot in the end like the L series does, is 12mm where it passes through the pump body & 1/2" where it press fits into the rotating pump element.
A second A series pump body is then sacrificed to make a spacer to extend the length of the A series pump as the L series parts are the same diameter as the A series parts, but longer. A billet spacer can be used instead if one was of a mind to make one.











Option 3 is to secure for yourself one of the factory competition oil pumps. These are a bit hard to find since they didn't sell a lot of them but they are around as I now have two of them & another forum member has recently secured one also, all within the last year or so.




Just as an aside, the majority of Datsun race engines use a stock oil pump because the volume & pressure output are regarded by many to be quite adequate.