If you can find some high-carbon steel, that should last longer, but may be more expensive if you have to actually buy it.
Heat-treating it will also make it harder - basically, get it hot (over 400 F) and then quench it with oil or water.
A good source for heavy steel is the highway department. You can sometimes get old plates (the big square ones they put on the roads to cover trenches, etc.) if they are getting rid of damaged ones.