...it's one of the most common resources, just increase your perception skill and walk along some water and you are bound to find 2-3 spots (per map) to harvest it... and pretty much every map has atleast 1 body of water.
I mean who ever used that spot?

You get stone from mining so I find myself with 99 stone easily without trying... actually the same counts for clay for previously mentioned reason.
run around the world for a week (in game obviously) and check your traps along the way... nothing else is needed to get 99 Clay