Any stiff metal object that has a sharp edge will throw decent sparks. The key is having an object that is rigid as well as having a well defined sharp edge. Whether it's the spine of a knife, piece of tool steel, or even a piece of sheet metal you get the idea.
The sharper the edge the striker has, the easier it will be for it to bite really into the ferro rod, the stiffer your striker is, the harder you can bear down on the striker to help rip out larger slivvers of the ferro rod to create larger longer lasting sparks.
One of the best strikers I've used is the corona carbide sharpener. It has a really square super hard carbide edge that just shreds apart ferro rods. It have a nice longer handle about 3" long that can really grasp and apply leverage. On the flip side, the down side of having a really effective striker is the obvious accelerated wear on your ferro rod, but to me ferro rods are cheap. Quick fire is priceless.