Hi.
Do you know of a vendor for soldering rosin flux, not paste, not liquid, not in-core, not modern, not foreign to U.S. ?

 

I am in Canada so MG Chemicals covers my soldering chemical needs.
they have catalog on soldering flux:
https://mgchemicals.com/downloads/category-data-sheets/CDS-Soldering Flux.pdf

you seem to be looking for soldering paste (8342)
https://www.digikey.com/en/products/detail/mg-chemicals/8342-50G/6612614

 

How about this?