A pea eggplant is, as you might have guessed, a very small eggplant - more the size of a large marble than a pea, in truth. They're typically cooked whole in Thai soups and curries, and pop in the mouth when you bite them, releasing a burst of bitterness that combines nicely with richness or heat in the dish.
There's a picture of some on the right-hand side of this web page.
We recommend you try the specialist south-east Asian grocers at the Mawson shopping centre. I've also seen them from time to time at the fruit and veg place at the Canberra Centre. Simply Fruit, I think they're called - they're usually pretty good with obscure Asian produce like banana leaves and birdseye chillies. Failing that, you can substitute the smallest eggplant you can find, or slices of small Lebanese eggplant. The effect won't be identical, but it'll be pretty tasty nonetheless.