Darkwraiths in lower New Londo Ruins drop regular slabs, Chaos Eaters in Lost Izalith drop red slabs, Moonlight Butterflies in the Crystal Cave drop blue slabs (but prepare for a long night as the drop rate is INSANELY low), and Pinwheels in Tomb of Giants drop white slabs.
If you need regular slabs, there are 3 guaranteed throughout the game. One from Stray Demon on the second trip to Undead Asylum, one in a chest next to Kalameet in the expansion area, and one from completing the Siegmeyer quest line and having Siegmeyer survive. You might also get one from killing Crystal Lizards in the Great Hollow. The rest have to be farmed from Darkwraiths.
Red slabs can be farmed from Chaos Eaters in Lost Izalith, and there's one in the Chaos Eater pit, in a chest somewhere at the bottom.
Blue slabs can be farmed from Moonlight Butterflies in the Crystal Cave, although the drop rate is extremely low, and there is one to be found in the Crystal Cave by taking a secret invisible path to a large crystal pillar on the far end of the cave. Recommended that you use Prism Stones to more easily find the path, as it can be deceptively easy to fall off.
If you intend to farm the butterflies, stock up on coffee.
Finally, White Slabs can be farmed by killing the Pinwheels in the Tomb of Giants, right before the fog gate leading to Gravelord Nito. These respawn unlike the boss from the Catacombs, so kill them, warp to the bonfire, run back, rinse and repeat. There is also a single white slab in the same room as the pinwheels, in a hidden tunnel towards the back.