Two metal O-rings between the manifold & the A-pipe. One donut gasket between the A-pipe & the cat. I got stuff like that from A&H just by calling & describing what I needed.

At the cat I've used hardware store stainless bolts, fender washers & re-used the springs.

Let us know how the welding works. When mine started buzzing, I used the excuse to buy a header. After putting that in, it looked like I could have got the A-pipe fixed by tacking the inner & outer tubes together at the outlet. Don't seal-weld it all the way around, because the space between the tubes needs a vent.