You can get 3M wet-o-dry just about anyplace - hardware store, home depot, menards etc. I'd suggest a medium grit, like 180-200 grit, and use it wet.
If it's way out of true, start with a coarser grit, and finish up with medium or fine.
You can use a little of the orange permatex on an intake gasket, but just the thinnest smear. If you need more, then the parts need truing.
I would not torque a manifold flange that uses a thick gasket more than 4-6 ft/lbs, assuming 6mm bolts. This is one of those places where the "mechanics feel" is very helpful..