I've replaced the manifold studs on each engine I've had as I would rather not be in the situation of having sheared studs stuck in the head and having to worry about how exactly I'm going to remove them. It doesn't take too long and it's a case of being better to be safe than sorry.
As for where to get them - refer to what Geoff said. I got all my studs from bursons and the best way to ensure it's the same thing is to take one of the studs in with you to get them to compare it with and make sure the thread, length and diameter of the stud is the same as the original. I wouldn't bother going to Nissan for studs like those as they'll often charge too much for what you can often get much cheaper elsewhere. That said for more critical items like engine seals and the like, I've preferred going to Nissan. The studs were marked as 'Nissan' so there's a good chance that they used the same size/thread of studs on several models.
I got the nuts from an exhaust place as Bursons didn't have the type I was after. The ones I wanted had an integrated washer which was perfect for the surface area needed on the flange of the extractors. Look up the yellow pages and do some ringing around if you're that desperate for them.
---dens
