"Audio Interfaces" come on many types. With a better understanding of precisely what you are attempting to connect your iPad via USB-C, contributors may be better able to provide definitive guidance. For information, the USB-C cable supplied with your iPad is a USB-C data cable.
Perhaps you are simply looking for a simple USB-C to 3.5mm Audio connection that incorporates a DAC? There are many from which to choose, including Apple's own:
- Apple USB-C to 3.5mm Headphone Jack Adapter:
https://store.apple.com/xc/product/MW2Q3ZM/A

If you're looking for something similar with better technical specifications, you might consider one of the Audioquest Dragonfly DACs - although you will also need a USB-C to USB Type-A adapter:
https://www.audioquest.com/products/dragonfly-red