Not sure what you mean when you refer to 1-1/2 elbow??? What do you mean by '3/8 fine thread'???
Assuming you are talking about the water outlet fitting in the fresh water tank that hooks to the line that goes to the inlet side of the pump...
Water supply lines in US products use NPT, National Pipe Thread, sized fittings.
When I replaced the 90 degree elbow in our tank, I used a 3/8 NPT, Street Elbow, and then screwed in the hose barb fitting into the elbow...to accomplish the same thing as a one-piece fitting. These fittings are common place in the marine industry...perhaps you could find them a boat supply place if you can't locate at large hardware supply.
West Marine is on the Net, and probably ships up your way...check them out too.
Good luck on your search.
Ray & Pat; Morada, CA