From soil. From the surface of the planet. From dead organic material.
A sudden worldwide flood would significantly erode, i.e. wash away, a large quantity of soil. When this happens, the soil particles would become suspended in the worldwide ocean. Over time, some of this material (along with dead marine life)would settle to the bottom and form a fairly uniform layer of material... you can see this occurring in all the oceans of the world today.
Also, assuming the floodwaters receded through evaporation*, the soil still left in suspension (which would be considerable) would remain after the water itself was gone, leaving its own uniform layer... try Googling pictures of the Bonneville Salt Flats if you want an example of what this might look like.
When you look at a cross-section of soil from any place where there used to be a lake, sea or other body of water, there's a layer of marine sediment present. If the entire planet had been underwater, we'd expect to see a layer of marine sediment anywhere we cared to look... but we don't. We only find marine sediment where conventional geological theory says that there was a lake, sea, ocean or the like, and never anywhere else... and the marine sediment that we do find is only consistent for that ancient body of water; if all the sediment that is there came from one global body of water, it would all be similar... it's not.
*If you believe that the flood waters were pulled up through "windows in Heaven" or something like that, I can't help you... if some supernatural mechanism did collect all the water without vibration, I suppose it's conceivable that any suspended solids got taken with it, but this would only affect the thickness of the sedimentary layer, not its outright existence.