Whatever you can get. (those wood slats off lattice is great for practice.)
Mild steel (1018) from the hardware store works, but they want waaaaaay too much for it.
If you can find a scrap metal place and ask for some mild steel 1/8" by 1" that's probably the cheapest you'll find. But it won't make a knife.
Admiralsteel.com sells 1084 in that size: 6 feet for $6.50. The shipping is more than the steel. However, when you're done if you like it you can harden it up and make something.
Steve