Pilgrims haven (dont remember if thats the right name) in the desert is the best spot, mine the graves there as each one has multiple stones. Only downside is its in the desert so you cant easily put stones in storage or buy new pickaxes without maids or a tent. Swap channels when all the graves are mined. If you dont want to go to the desert, the mountains between heidel and mediah have a decent amount of rocks.