You can actually make one of these keys using metal wire (around 20 gauge) available at arts and crafts stores such as Michaels or DeSerres. Here’s a picture of the one I made for my model 700 pull station:
The tip of the key (at the far right on the picture) has a width of eight millimeters; it can be quite tricky to get just the right size while you bend it as the tip has to be small enough to fit through the slot at the bottom yet big enough to push the springs inside the pull station apart and around the metal peg.