My experience has been that a harder steel works better than a softer one. I'm no expert, but I've used fire steels, both my own and one I bought several years ago, to light a few fires. When I made some out of spring steel, I wasn't getting quite the same quality of sparks as from the purchased one. Use a file, you get a lot of sparks, but only if hardened. Like really hard.
That said, steel, whether mild or of reasonable carbon content, is fairly cheap. A garage door spring will only set you back a little and if you get friendly with someone who works at a recycle yard, he might tell you when some springs come in and you can get them for scrap prices. There's legitimate concern about stress cracks in used springs, so you might not want to use them for knives you plan to sell, but as far as strikers, that's less of a concern.
Some time back I read a claim (so this is third-hand information and therefore only a suggestion of where to start investigating, not actual claims of truth) that if you overheat the steel for a striker, it gives better sparks. As in, don't heat just to nonmagnetic, but up into the orange range like you were going to forge it and then quench from that temperature. I can't tell much of a difference between strikers I've heated to critical and strikers I've heated well above, but I only have a few to go on and not as much time as I'd like for experimenting. I share the thought only in case it helps others.
-Patrick
