Here in the UK we substituted Penguin biscuits: they worked OK, but not as good as Tim-Tams.
Penguin (biscuit) - Wikipedia
From the wiki, I see that:
The Tim Tam, produced by Arnott’s in Australia and first sold in 1964, was based on the Penguin.[2] Occasional media references include tongue-in-cheek debates over which is the superior biscuit.[3][4]
And they have the advantage of a ‘joke’ on every wrapper:
What type of pasta do Penguins eat? Penguine
What do Penguins wear to the beach? Flipper-flops
And so on…