I don't know how the New C64 implements USB HID, so I can't guarantee compatibility, but here are the options I'm aware of:
- Stepstick.pl appears to still be selling C64-to-USB adapters if you want something pre-assembled. (They sell two products, but I couldn't figure out how to set the "English, please" cookie without linking to a specific product)
- Commodore4ever is also selling an adapter that they also offer via an eBay listing as I'm writing this.
If you need something that appears as a USB keyboard and you're handy with microcontrollers, here are some options:
I'm not sure what kind of HID device it presents as, but Markus Gritsch also did a design using a PIC microcontroller.