Aussie- the company is known as Tuf-Wear Manufacturing Inc. If you Google search them, most of the results come back to their boxing gear.
Their football equipment is sold through regional distributors and they don't appear to deal direct with customers.
The address and number I find is
1001 Industrial Ave North Platte, Nebraska 69101
(308) 532-0187
Having worked in sporting goods many years ago, my guess is this being corporate HQ, you may get the runaround but somebody there should be able to put you through to someone, somewhere who might can help you with parts. I took one of their Dial-A-Down markers apart for our local high school several years ago. I can tell you they are made of almost 100% plastic. You will probably be better off and easier to just purchase a new one. I understand that shipping it to Australia will probably cost more than its worth.
You might consider jury rigging something yourself or go for a cheaper alternative marker. I prefer the metal ones that flip myself. Down markers get banged up quite a bit and frankly, the all plastic Dial-A-Down isn't going to handle the workload placed on it forever.