They can be found on ebay and amazon. I paid $5 for a bag of 10, with free shipping from US seller.

They crush and seal nicely without being over-tightened.

And they fit the drain plug well.

A bag of 10 should last 10 yrs or 80000 miles, per the manual.
I've used 5 in 7 mos & 3200 miles.
Fish