Which stove are you having trouble with? To be honest I have no clue on the thread size, but maybe try these guys who are the distributors of MSR gear in Australia:
http://www.spelean.com.au/welcome.html. I've had to contact them in the past for a spare part for my Dragonfly and they were awesome, really friendly, helpful, and sent out what I needed at no charge.
Another suggestion would be to see if you can find an old Expedition Maintenance Kit for your stove. I know most have spare jets with them, so you could find a replacement for your stove that way too.