You don't need a lapping stone. Sandpaper on your countertop/tile/etc is fine. The lapping stones are mostly for convenience/efficiency.

They do make life a lot easier, but if you're looking to save money for now, don't sweat it. Do expect your swaty to be a bear to lap the first (only?) time, though, and the nortons take a while the first time as well. Once everything is flat, touchup lappings are a breeze save for the inconvenience the sandpaper adds.

Eventually, you'll get fed up with getting out and setting up the sandpaper and buy a D8C