Import wreckers like Just Jap can get them, not just Nissan.
The water pump is the same across all the RB engines. The only one that is different is the RB26 N1 water pump. So, if you are after one quickly, try Holden, ask for one for a VL Commodore.
Some RB30E parts from Holden are still cheapish, water pumps may be one of them, and the dealer may carry them in stock.
Or just for a laugh, call Nissan again and ask for a water pump to suit a 1986-1990 Australian delivered R31 Skyline. If they say they have heaps in stock, get one of those..... the same again.