For the most part, the harder the metal is, the better the feedback but lower the "springiness", and vice versa. So there is obviously a middle ground here. IMHO the characteristics I've found in these razor blade replacements are better than the spring steel offered with commercial lock picks.
While I have experimented with windshield wiper inserts, hacksaw blades, and street sweeper bristles, I have found all of these far too ductile to work with. If this sounds familiar to anyone, give this a try! Besides... It's cheap

I apologize for the picture quality. These were done with an Ipod.

These are the replacement blades I bought. Any brand will do, but I recommend buying as cheap as you can get


As you can see, there is still a fair amount of flex in the metal. Far more than I think any experienced or inexperienced lockpicker will need. And the scored edges for the break points won't break unless EXTREME pressure is exhibited on the blades. So if the blade happens to break while picking....you were probably doing something wrong in the first place.

The pick flexes naturally.

And returns to it's original shape. I have yet to distort these picks.

These are the only picks I've made so far. I actually like the signature "break points" in the picks. IMO it adds a little bit of style.
I hope this writeup was helpful!