There are only four slabs in The Ringed City, not 6.
-Lapp will tell you there's treasure in a cavern in Earthen Peak, right at the the swamp before the third Angel. It's guarded by a Harald Knight in the left corner. If you die or miss it and come back to Lapp he will gift it to you and you'll find the Harald Knight slain.
-Beating Halflight, Spear of the Church earns you a slab. (You can reset the fight, but a slab is only awarded on the first kill)
-Shira will give you a slab for killing Darkeater Midir. Alternatively, you can get this slab from her by killing her after the final boss in the chapel to the right of Filianore's Rest's bonfire.
-Kill the Ringed Knight wandering in the final area alone to the left for one.