It just depends on what you want, and maybe what kind of tender you might have or take or find. You can spend from $7 on a Swedish firesteel mini, up and beyond $25 for the Aurora, both work.
I have the Aurora, with the carbide/cobalt blade, and it works just fine. I guess I wanted to always have the blade so I didn't even need my knife(as a striker).
I also own the ESEE fire kit, it's got a watertight storage compartment to hold some tender, and a button compass, it works great as well.
Bark River and plenty of other knife makers can make you one with a custom handle, or one that matches a particular knife.
Others may chime in regarding the Blastmatch, nano strikers, or others. Camping Survival, Knife Center, and others have many to choose from, even plain "blank" ferro rods that you can attach a handle to.