Repco and Coventry can get them.
The company that makes them is Garmax...the part number is JS100 or something like that. If you are in Brisbane you can call repco springwood on 07-3208-5000, ask for Andrew or Lee and they should be able to sort you out.
If you are doing a turbo upgrade I would assume you need the Turbine housing gaskets, not the manifold...unless you are doing the manifold one as well.
The turbine housing gaskets are T3...or the HKS GT2530 also comes in T25. But Skyline is T3....any good performance shop should have them on the shelf. Also T3 turbine in is different from T3 turbine out so you'll need two different ones.