Unfortunately there is a bit of a hole in the 650c offerings. You're probably looking at kid's bikes.
I saw a little Giant, in 24" (520 ISO), I think. They may have 650c.
I don't see one listed.
There is a Fuji Ace 650c. It is one of the cheapest of the kid's bikes, but perhaps could be upgraded.
Felt apparently has a few too, a bit of a step up from the Fuji bikes.
Trek used to make a 650c Madone, but I think it has been out of production for a number of years.
There is also a 650c Trek 1000.
I'd keep an eye on E-Bay. There appear to be a few Trek 1000's that are showing up. Here are a few of the more interesting bikes I see a the moment. Or, wait to see what shows up over time.
This TiCycles bike looks like it would be a SWEET bike.
https://www.ebay.com/itm/42cm-Ti-Cyc...W/123400641736
Also this Trek 5000 (early 2000's)
https://www.ebay.com/itm/Trek-5000-O...d/264030595759
Trek 5200, also early 2000's.
https://www.ebay.com/itm/Trek-5200-W...3/132841730867
Giant OCR
https://www.ebay.com/itm/2007-Giant-...e/132827491161
Pretty old Cannondale. Missing a lot of stuff, but all the important stuff is there, and you could upgrade to your own specs.
https://www.ebay.com/itm/CANNONDALE-...S/292814458740
Felt F95
https://www.ebay.com/itm/Felt-F95-Jr...W/113299038357
I'd avoid the TT/Triathlon bikes, or at least be very wary of them as they may well have a bit wonky of geometry, sized for large riders with low bars.