In case anyone else needs this:
I found a replacement by searching for Shimano Claris Braze-on triple 8 speed front derailleur. It cost about $18 with $20 shipping from Japan on eBay and/or Amazon or UK stores. It replaced the Shimano Sora the mechanic recommended.
The part name and code is:
Shimano Claris FD-R2030-F Front Derailleur 3x8 Speed (Direct mount / Braze-On) EFDR2030F
And it should work for replacing older bikes' derailleurs. The amazon reviews are helpful:
https://www.amazon.com/SHIMANO-FD-24.../dp/B00CABR500