In this picture I used a valve spring from a 1991 Ford Escort. I'm not sure this is ideal for this purpose, but I had a small pile of them in my parts bin so I figured it was worth a try. Since brake feel in sim-racing seems to be more about personal preference than reality (read some threads on that subject... it's a rabbit hole) I would guess you'll need to buy many springs and experiment a bit.

Maybe you could find a local to you industrial or hardware supplier and pick up a few?