Regulators don't normally come sized with 3/8" barbs. I'm not saying they don't exist but I went to Taprite's site to see and didn't find a replacement valve with that barb size.
You looked in the wrong place
https://www.beveragecraft.com/primary-co2-regulator-micromatic/
Lemme tell ya getting 5mm ID EVAbarrier tubing over that bastard was a pita but I got it done with heat and a swaging spike...

Cheers!