For entertainment only, I made my own rosin flux from blobs of dry pine sap disolved in isopropyl alcohol, filtered to remove impurities and heated to concentrate the rosin. It worked well enough but it's abietic and pimaric acid concentrations were unknown. I think I read somewhere that commercial flux is typically made from the resin of southern long leaf pines.
If I were going to do it again, I'd add glycerin to make it more anaerobic and a small amount of zinc chloride as an activator.