What size ball bearings for FSA Orbit DL headset?

The bearings in my FSA headset have worn out. They are cages ball bearings and I can't find any direct replacements for them. My local bearing shop is not so local and it would be really inconvenient for me to try and go there for help.
So any body know what size ball bearings to replace them with? Do I just measure the size of the caged balls and use that? or ... ?
many thanks in advance ...
So any body know what size ball bearings to replace them with? Do I just measure the size of the caged balls and use that? or ... ?
many thanks in advance ...
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Parktools :?:SheldonBrown
or will any correctly sized bearing do the job just as well?
Grade 25 is suitable.
Parktools :?:SheldonBrown