The OP doesn't specify head tube diameter, threadless vs threaded, or whether it's going on the steerer, crown or stem.
Since this is the RBW Group and since most Rivendell bicycles are 1" threaded headsets with plenty of steertube, I'll assume that's the type the OP needs. I believe the DiaCompe ones run a M5 x 0.8mm threaded barrel. Cycles Grand Bois have them, and maybe you could find them elsewhere.
Of the three on that page I'm talking about the second or third one. Not the first one.
El Cerrito, CA