Lots of amateurs who think they'll make their own "special" try using an old file. These are generally "glass hard" and very difficult to temper so they're not brittle, and once you expose fresh surface they usually rust quickly.

Worn out power saw blades make a fairly decent blade for an amateur job, and "gunsmith temper" is often good enough. After a quench harden, you lay the blade in a small pool of engine oil, light the oil and let it burn off. Flame temperature is about right, without forced air, to temper-anneal small "spring-steel" objects below "brittle" stage, and may be "close enough" for a home made blade. (Do this outside - it smokes a lot.)

For a pro, a "minimum buy" from a metal supplier probably would keep you well supplied for a decade or so, unless you're into serious volume. Choosing what specific material to buy for the next few years can be tough though. Finding what you want (or something close) at a salvage or end-lot seller may be even tougher.

I'd usually check The Yard first; but it's local for me and I can wander in and poke around. They have lots of stuff that's not in the catalog.

John