What size gasket do I need to buy for my invidia mid pipe 70mm?

 

header down before the Y-pipe is 60mm. after the Y-pipe is 70mm.

http://www.invidia-usa.com/Products.aspx?ProductID=195

you can try contacting them to see if they have gaskets available, though (iirc) you should have two usable gaskets for the header-down section (unless i'm completely wrong).

http://www.invidia-usa.com/Support.aspx