I have used "cabinet scrapers" in the past when needing larger pieces of high-quality stainless spring steel. You can find them at most stores that carry higher-end woodworking tools. (The scrapers are normally used when refinishing wooden furniture.)
They have the advantages of being stainless, fairly well hardened, and very thin: I find them great for making custom picks for locks with tighter keyways. The scrapers are usually 2"x6", and a pair of 0.01"-thick (0.25mm) plates should be about $10. (That should be enough material for 8 or 10 picks if you're sticking to Southord-like dimensions.) I usually get mine from
Lee Valley Tools, but I've seen them elsewhere.
Just be forewarned: the harder stainless makes good picks, but it takes a lot more work to file into shape.