So you want OEM quality, but not OEM price? - You can order them from somewhere like FRSport or maybe Eric's... but then you need to wait for them to be shipped from the US.
I didn't think they were really that expensive from Nissan, and I'm pretty sure you the rocker cover gaskets as a set.
There is no flat gasket between the down pipe and the turbo elbow; you'll find it has one of conical compression ones on it. - Any exhaust shop should have them.