How do I get a driver for HP SMAR Tank Plus 651 for my iMac running OS 10.13?

Hi,

You may not need an additional driver, providing that the printer and the computer are connected to the same network (Wi-Fi router). The HP Smart Tank Plus Wireless 651 All-in-One Printer is mentioned in About AirPrint - Apple Support as a model supporting the driverless AirPrint technology.

Reset the Mac printing system to solve a problem - Apple Support

Add a printer on Mac - Apple Support

Go to the HP Support site for the printer,

https://support.hp.com/us-en/drivers/selfservice/hp-smart-tank-plus-650-wireless-all-in-one-series/18695943/model/31017110

Make sure the operating system selector is set fo macOS 10.13 High Sierra.

Download the HP Smart app and run it , it should download the necessary drivers for the printer.

There is also a firmware update you can download and then install.