Ahhhhh for some reason I was thinking threads in the pump (as in "big electric") instead of the carb. Those are proprietary fittings. SOMEONE may make the one you seek, I just don't know who.
So I"m guessing the line from your pump is the mechanical pump. I've cheated in the past, some 5/16 hose you can stretch over 3/8, and some 3/8 hose you can clamp down on 5/16
