Apologies for bringing up another cold case. I try to limit myself to one case at a time.
Headset bearing, a.k.a Angular Contact Bearing(ACB), are 9 out of 10 made by TH industries (FSA).
Each ACB bearing has engraving on its side, usually coded as TH -xxx or MRxxx, if luckily it’s still legible from corrosion. This code is the KEY to find replacement. It tells the property of the bearing: angle, diameter, etc.
Use that KEY to buy/inquire replacement. Sometimes different ACB shares the same property. So using the deciphered information to search replacement is a further alternative.
Alternatively, a bicycle bearing specialist in Australia, DIY MTB is another secret tool of the trade. They stock a good range including some awkward TREK and Specialized sizes.
Headset bearing is a worldwide headache in cycling. It never gets fully standardized. A so-called 1 1/8" ACB has more than 10 varieties and not cross-compatible, not to mention those modern oversized ones. It is why most shops rather sell a full set to avoid future drama.
In Taipei there was one mechanic-based Vanden Cycling who specialized in bearings but he now moved on to coaching business and moved to Hualien.