I was going to say why not try Grant, its who I did my OFTEC with a long time ago and they've helped me with spares in the past.
I've tried finding a part number online but can only find one for the whole extension kit unfortunately.
Two choices, either buy a few kits and replace the whole lot with your normal mark up price or buy one, remove the washer and see if a local nuts and bolts company to you have them or can make them up for you. We have one of those companies in my town and they've helped out many times with various requirements.