Stone selection.  

 If you can't get limestone, then sandstone will do. A piece of stone can only be assessed on it's own merits. Look around and find the piece of stone you want ( maybe even take a chisel to it) before you part with any money.

Rocks in the riverbed are not usually any good for carving. Can you believe that in some cases they were put there because they had no use other than stopping erosion. Stone in water either gets harder or it dissolves. It's one of lifes mysteries.

Give yourself a headstart
These are pieces of stone. 

Lumps of rock
These are lumps of rock