They are no longer made, but they can still be purchased. I buy them at peachparts, at this link:
SPECIAL ORDER BOSCH H9DCO, these are "NOT" resistor spark plugs ''ONLY AVAILABLE WHILE STOCK LASTS'',0031590403 - PeachParts

They are currently $4.90 each.

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Streetside: 1987 Mercedes 300E Petrol Blue/Green
2014 Ford Mustang Grabber Blue
1999 Ford Ranger Green

Dockside: 1968 Eltro 17
1985 Silverton 34C