9to5Mac published a story that the Mac Pro Afterburner Card is now available as standalone purchase.

Apple makes Mac Pro Afterburner Card available as standalone purchase

Sneaking in with the major product releases today, Apple has started selling its Mac Pro Afterburner Card as a separate purchase. When the Mac Pro first launched, the Afterburner Card was only an option to purchase when configuring the machine as a $2,000 upgrade.

Apple makes Mac Pro Afterburner Card available as standalone purchase - 9to5Mac