Yes there are now some new sanitary thingermejiggs from abroad which use a fine 3/8th thread which you stand no chance of finding. They do usually come
with an adapter to 1/2" which should have been fitted by the installer. So far I have not been able to find any tap connectors with this foreign fine thread. I dread to think how I'm going to cope with emergency call outs when these foreign thingies go wrong 'cause the mechanics behind them defies logic, and to my knowledge no parts are available. s'pose we'll just have to sell you a whole new one when that day arrives. Or maybe they'll work perfectly forever? Yer right!

Any how you'll most likely find it takes a 1/2" tap connector.

Watch those flexies. The ones with rubber washers are OK but the crappy bit of cardboard some use for a fibre washer wants rooting out with a fine screwdriver and a proper replacement putting in, trust me, if you don't want to be decorating the room below in a fortnight.