Stupid question

Hobby store, maybe. My take on the assignment would also be sodium (or potassium - even more violent), but then it’s been 40 years since general chemistry.

If all else fails, tell him that you were concerned with safely transporting the elemental sodium, so you combined it with chlorine to render it inert. And hand him some table salt (Salt is NaCl - sodium chloride)

Anything that causes a fire in water is too unstable to be sold in normal stores. Just touching it would make it react with the moisture on your hands.

http://www.youtube.com/watch?v=JD85OUkEKKw

You could try ordering it online from a chemistry supply website. Or those in the video got it on ebay. I’m not sure but I think you have to store it in oil just to keep it away from moisture.

Magnisium? I seem to remember something about magnesium having explosive properties when combined with water…

Something that can cause fire when put in water? Hmm. How about a fisherman? ;)

More seriously, though, you might find it easier to obtain quicklime: http://en.wikipedia.org/wiki/Quicklime

Along with Sodium, group one elements on the periodic table.

My chemistry teacher showed it to me, how the sodium was reacting on the water, it sparked with fire and spinned around on water until it dissapeared. A very cool show! And you only need a very small piece of sodium (like your little finger nail size), and they keep sodium soaked in the oil to prevent it exploding. I dont know where to buy though,sorry doesnt help :p. But it must be not very easy to get it since it is a dangerous substance.