Does anyone know if there are flat triggers available for the G48 yet? Does the G17 trigger fit?

A G17 trigger shoe should fit, but the typical “trigger with trigger bar” will not. Any trigger with trigger bar assembly for a G43 should work in your G48. Most of th major manufacturers of aftermarket flat triggers should have one available, just pick your favorite.

Overwatch Precision has drop-in flat face trigger assemblies.

Any trigger shoe can be retrofitted into a G43(X)/48 trigger bar if you remove the factory shoe and install it yourself.

Apex.

Overwatch Precision DAT polymer trigger. Have one in my 43x and 48 they are awesome and cost less than 60 bucks.