I think Rhys and Ron, and George have covered this pretty well, but I'll toss in my comments as well.

I personally has to replace several manifold studs on my new B20. A quick visit to your local Ace Hardware (or equivalent) should yield you several "automotive studs." They are usually in a box in the nuts and bolts section and run between $0.80 and $1.50 each. Pick up some red loctite thread lock to help get the new stud to stay in.

When I was looking, I could only find 5/16" with coarse threads (18) on the head side and fine thread (24) on the manifold side. This is all that's easily available. I replaced all nine of mine with matched studs, but had to pick up a set of 24 pitch 5/16" nuts to match the new studs. I went with Grade 8 nuts and washers to supplement the thicker volvo washers.

BTW - I've lost a number of manifold studs between the three motors I've been working on. Only a one actualy stripped. The remainder broke off either as I tried to remove the holding nuts, or before I got the motor. Seems to be reasonably common for high milage motors...