The new fourth generation iPad Air will be supplied with a 20W USB-C Power Adapter - that supports USB-PD fast charging. Using the supplied Power Adapter will allow your iPad to charge at its fastest supported rate.
If you would prefer to use a standard USB Power Adapter, charging of the iPad will be considerably slower. If this is your preference, you’ll need a USB-C to USB3 Type-A cable. These cables are readily available; here is one option marketed by Apple:
mophie USB-A Cable with USB-C Connector (1 m)
https://store.apple.com/uk/xc/product/HN892ZM/A