Pete, I've no idea where you could buy one of these but have you considered having one forged. The ones the Japanese turner is using look hand forged.
I bought a Nic Westterman spoon carving blade for £35, you want something similar but stronger and with a longer handle, but are capable of sharpening it yourself. So more costly one way and cost savings the other.
Try asking him, or someone else who does forge work. You may need to provide a reasonable working drawing.