Odd question about hydraulic level gauges

Hi guys, I'm stumped here, looking for help. I have a customer who wants to replace his level gauge on his tank. It's a regular 5" gauge, no biggie there. My problem, or rather his problem is the brass adapters on the tank that adapt from 3/8 npt on the tank to 1/2-20 bolts for the level gauge. He wants to use a level gauge that has 1/2-13 bolts instead of the 1/2-20. The brass adapters that are in there now are 3/8 npt to 1/2-20. I can't find any adapters that go from 3/8 npt to 1/2-13. Does anyone know where I can find anything like that? Thanks!