The steel at TSC is, I believe, ASTM specification A569 (a low-alloy carbon steel with no more than 0.15% carbon) or A-36 (structural steel). If memory serves, they charge about $2/pound. Good for a lot of basic jobs, but a bit pricey.
See if you have any steel suppliers, fabricators, or spring shops in your area that will sell you "drops" (offcuts from larger jobs) for cheap. (There's a steel place a few blocks from where I drop my son off for school that sells me their drops for about 75¢/pound.) Any kind of new knifemaking or tool steel (other than drops of 5160 or the like from a spring shop) is probably something you'll have to get through the mail.