After contacting a Perth company, who seemed to be the only suppliers of LaCimbali Domus parts, for a new group head gasket, I was quoted $27.00 including delivery to Canberra. :o
I measured the old O-ring style group seal, and found its VERY close to an O-ring that any bearing shop will sell for about 50c.
You need to ask for a #228 O-ring. It should be 2-1/4" diameter, and a 1/8" diameter section.
It is marginally smaller than the original, but after 1 month of testing at 6 shots/day its held up well.
Save yourself $26.50!
This is for the white LaCimbali Domus, not the HX machine.