Olanigw is correct; any high carbon steel file will work. It is easy enough to pick up a cheap used one at a pawn shop. One of the things you might want to do if you use a file, is grind off the edges of the file so that you have a smooth edge. As you strike the flint and steel together, a small piece of hot steel will be shaved off. That is what you see as the sparks. It is easier to get this to work with a flat, smooth surface.
If you decide to buy a steel, there are quite a few places that sell them. They are easy to find on the internet. The place that first comes to my mind is Crazy Crow Trading Post.
You will need some charred material to catch your sparks on. Most people use 100% cotton cloth for this. An old T-shirt or pair of boxers will foot the bill for this. Cut the material into long strips about 2 inches wide. Fold loosely, and place in an Altoids tin. Put the tin in the fire (a gas grill or propane torch will work), and set it sit until it quits smoking. Remove from heat, and let it sit for about 30 minutes before you open it. When you get ready to use your kit, remove a piece of charred cloth and place it so that you can direct a spark from your flint and steel onto the cloth. People do this in different ways, but I have been most successful by holding my flint in my left hand with a small piece of char cloth on top of the flint. I hold the cloth close to the striking edge with my thumb. I use a downward stroke with the striker in my right hand. I can get a spark to land on the cloth and the cloth will begin to smolder. Blow on the smoldering cloth and place into a small bundle of tinder such as grass. I like to use shredded cedar/ juniper bark for my tinder. Continue blowing into it, and it should light.
Once you master the use of char cloth, you can start using charred, punky wood for the same purpose. The wood is a little more difficult to use, but the technique is the same. I keep a strike a light bag tied to my quiver. The photo that I am attaching shows one of my strike a light bags, flint, steel, charred punk and a fire horn. The purpose of the horn it to snuff out the punk and preserve the thin, crusty layer of char that is needed to get the wood to ignite.
