Exhaust manifold studs are NOT M8

Definetly an M10 bolt.

The helicoil kit should come with everything you need other than the correct drill bit size.

I'm pretty sure they dont use a "next size up" on the tap either, believe they're specific to the coil inserts which are the correct size for the thread you're trying to repair.

As for sources, autozone and advance both carry quite a few helicoil kits and inserts, napa has them also you've just got to ask, grainger is another good one.