Dan's had good results from seed packets bought from an Australian seed company called Green Harvest. The seeds are under the name Cucirbito Pepo (gem squash). The website address is http://www.greenharvest.com.au/.
Brnton has bought packets of gem squash seeds from Melbourne-based webstore http://www.diggers.com.au/.
Springbok Foods on the Gold Coast has been selling gem squash seeds for the last couple of months and currently have in stock. See www.springbokfoods.com.
Joslyn says that you can bring gem squash seeds in from South Africa, but you need to declare them at customs.
If you're looking for gem squash itself, they are available from Satooz http://www.satooz.com/ (thanks, Dianne) as well as Springbok Foods http://www.springbokfoods.com/ during season.
Linda says that her neighbour Glenn Jackwitz is going to be growing them on his Claredon farm this year. They will be planting in August so the gem squash will be ready in November. If you're interested in making contact with them directly, you can call Glenn's farm manager Simone on 0404 816 649.
Lucia let us know that she's purchased gem squash from the South African butcher in St Ives (Stanley Street Butcher). She scooped out the seeds and planted them, which worked like a charm.