These were fitted to every V6 D40, but a quick scan of eBay doesn’t turn any up.
Your two sources of parts would therefore be to contact an Australian Nissan dealer directly to see if they have any new-old stock, or contact a wrecker who might be able to remove the badge from a tailgate for you.
I’ve found a couple of D40 ST-X 550s being wrecked for parts on gumtree.com.au, maybe start there?