Its not necessarily a special supergun you need.
The extra harness usually goes between 3 extra buttons (the kick buttons) and the CPS2 board itself. It does not touch the supergun itself.
So basically the punch buttons are the ABC buttons that go through the jamma harness, the 3 extra buttons take a completely different route and go straight to the PCB.
So, all you need is a joystick which has inputs for buttons D E F (456).
You would have to talk to your supergun supplier (builder) so that they can accomadate this feature.
The harness itself uses weird parts, but I believe
www.jammaparts.com are looking into making them for CPS2 as I asked them just last week.
Just buy a cabinet from Capcom, it has all the buttons and harnesses already in it.
This industry has become 2 dimensional as it transcended into a 3D world.