Just take one piece of the exhaust to the hardware store and get the new nuts/bolts in something that
won't rust. It cost me less than $8 I think at an ACE Hardware. As for the gaskets, I just ordered 2.5" 2 bolt exhaust gaskets from amazon for a couple dollars each. Some go to Advanced auto and get ones made by Felpro. You don't need to order them from Cobb and they'll probably run you a bit less to source yourself.
Edit... for reference, these are the ones I used for my Cobb Exhaust:
https://www.amazon.com/gp/product/B01N6W17SN/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1