I'd remove another one, without breaking it off, take it to a good old fashioned auto parts store or a good hardware store and go to their DORMAN's division. A display with many drawers with things like this. I believe the studs are standard american size. Replace them all if you can get the others out and they're rusted like the one that broke. Coat with Neversieze before installation.
One note: It's possible that the sud that broke is a tad longer than the others because you're also clamping down on a tab on the water tube. Someone else will have been next to one of these more recently than I.
After you remove the old studs, run a tap in to clean out the threads in the head.