If your talking about using it for the lock spring, I know of one maker that uses the spring off of drum brakes, he just cuts a straight section and drills a hole at an angle in the but spacer to hold the spring. Seams to work pretty good. Not sure what the steel is, but I'd bet in the 1070-1095 range.
I think you'll find that there are two different methods of making a lock back. One has an integral spring on the locking bar. With this method, you'd obviously have to do the whole spring heat treat thing. In "how to make folding knives" by Clay, Lake and Centofante, one of them uses this method and the back bar is made of the same material as the blade (ATS 34 in this case). I don't know exactly how they handled the heat treat. Its not a spring that has to flex very far and doesn't have any complex bends so I think you can get off pretty easy with it.
The second method is to use a spring mounted in the backspacer seperate form the back bar. I think this looks like a much easier way to go. Using this method, most folks use a peice of work hardened 300 series stainless (available from TKS in a few different sizes for cheap). Its a flat leaf spring thats just stuck in a slot in the backspacer. It won't come out once the knife is put together cause its always under a load. This is kind of nice cause you have 2 ways to adjust the tension, first in the angle of the slot, and second by bending it a little bit.
Another variation of this is to use a coil spring (wayne clay does this in the book) which sits in a little slot in the backspacer and pushes up on the very back of the lock bar. Finding the right kind of spring could be a little more difficult cause if its wound too tightly it obviously won't compress enough to let the lock bar travel. But it should still be pretty easy to do.
Oh yeah, as far as whats the best spring steel, I don't know but I thought I'd throw all that out there cause I'm supposed to be writing a 4-6 page paper on Frederick Douglass's view of literacy and he's not around to ask, so I'm putting it off and needed something more fun to do :footinmou
My vote is for 1095 also. I've always done springs with a torch and I rarely have had any problems. I guess I'm just better than you guys You just have to pay really close attention to your temps and get it right the first time around.
You could always use Ti if it's not a visible part.