Planning to install my hr y pipe and a g35 coupe mid pipe on my 04 sedan this coming weekend but I have no clue what gaskets to buy.I need to know the sizes from the two y pipe sides (the sides that bolt on to the cats) and the gasket for the end of the y pipe that connects to the midpipe.

Jut re-use the stock gaskets on your car if they're in good shape. They're metal and should last the lifetime of the car if you don't live in the North or anywhere else that salts their roads in the winter.

They are a universal item. Any auto store has them $12.