Try Bike Tools Etc. or Loose Screws to see if they have replacement bearings. Otherwise your LBS could order them from QBP or one of their other suppliers.
Replacement bearings are often priced so buying an entire new headset is more attractive.
Thanks, I'll check those out. One unnamed online store suggested just packing loose bearings, but I don't think they were familiar with headset - that would leave no seals and the bearings would be too small. I think I ended up in Harris Cyclery's spam box.