Steel cut from an old hand saw was a traditional source. Also pieces of old bandsaw blades.
Old rip saws like Disston number 7’s or 8’s were good. I see them sans handles for a buck or two in scrap yards. They were thicker than saws made after wwI.
the band saw steel also was thicker than current versions, so saw mill band saws. Here in the pacific nw the supply is seemingly endless.
They were also a source for scraper blades.