I'd be happy to be proven wrong, but two suppliers have told me that this gasket is not available separately. There is not even a Volvo part number listed. Perhaps quality aftermarket suppliers may provide them, say Victor Reinz or Elring but I've not found one listed.
Perhaps it is my stubborn nature, but I cannot bring myself to pay for a new thermostat assembly when a simple gasket that should be available for less than ten dollars would solve the problem. The OEM Volvo gasket between the thermostat and cyl head (larger and more intricate) is available from multiple sources for less than $7, for Petes sake! I'll chalk this up as one of those many mysteries that involve P2 Volvo ownership.
Jim






