Silly question, but aren't jewels available in tons of sizes? Could you just use some gauge pins to figure out the OD and a micrometer to figure out the pivot size and order one? I see esslinger has an assorted set for like $13:
https://www.esslinger.com/hole-jewel-assortment-for-watches-100-pieces/
You MIGHT need some reamers to enlarge the hole in case you don't find one that matches perfectly (and it might be worn out, since why else would the jewel fall out?), but I would expect these to be available in a ton of sizes.
Side note question: Is there a mcmaster-carr type-site for watchmaking?