You can use leaves from any sort of deciduous tree, however most if not all of them should have dropped their leaves months ago so there won't be many laying around.
Peat moss or coconut fibre can be soaked in water to release tannins. Pure peat moss is generally less likely to have chemicals compared to coconut fibre.
Driftwood can release tannins that stain the water.
Most Australian native plants can be used but collect dry leaves that have fallen on the ground. Make sure they haven't been sprayed with anything.
Sera and Tetra both make a blackwater extract that you can add to aquariums to simulate the blackwater look.
