Summit Racing had the head studs in stock, but that was a few years ago. The price was a few dollars cheaper than I saw anywhere else. The part# for all L-series 6's is the same. They have a good website to for ordering from.
Anyone correct me if I'm wrong, but you will most likely have to find the thickness of the rings that came with the pistons, and the exact bore size, which is probably 3.425 inches. As for a supplier, I may get chastised for this, but Top End does sell them. They're not the most popular vendor, but I've had no problems.
I used oem manifold studs, but have seen stainless out there. MSA may have them. I know zcarcreations.com sells an entire stainless engine kit.
The head gasket will get you a variety of answers, but the best one to me, especially for a turbo, seems to be factory Nissan or Felpro with the thin wire copper o-ring setup.
EDIT: just checked zcarcreations, and they do in fact offer an exhaust stud kit on ebay, with a "buy it now price" of $38.95
http://stores.ebay.com/Restoration-Stainless
The only problem I can see is if you're using the FI manifold, you'll need the larger bolts for the top.