Keyboard for iPad Pro 1 generation 12,9

Apple no longer sells a keyboard compatible with that particular model.

However, pretty much any Bluetooth keyboard out there will work with it.

If you want a keyboard case, then look for ones compatible with the First or Second Gen iPad Pro 12.9" (2015/2017 models)on Amazon or other online retailers: iPad Pro 12.9" First / Second Generation Keyboard cases - Amazon.com

You might find the old Apple smart keyboard for that old iPad Pro on Amazon.

Brydge keyboards still make a keyboard for that old iPad Pro.

You can find Bluetooth keyboards/keyboard cases for that old iPad Pro on Amazon.

Best of Luck to You!