Blocks of magnesuim that come with a strip of imbedded flint web page are available in most sports, camping and big-box stores. These are actually knock-offs of the military types. There's enough magnesium in those to make hundreds of BOFs.

I actually prefer these old types strikers, probably because I'm used to them. So I can't give an opinion on these newer forms of sparkers. Anything that makes a spark will work....even a 9 volt battery and steel wool will work.