Most turners (at least the ones I know) use a combination of wood which they buy ready dried, and wood that they have "obtained" themselves and allowed to air dry naturally.
Normally logs in short lengths like those pictures would have end grain sealer applied to them to stop them splitting.
Every spare inch of space in my workshop is full of logs of various species in various stages of drying. I also have some larger cherry logs drying in a shelter that I built to keep the coal for our central heating system dry (I deliberately built it bigger than necessary so that I would have space for timber as well).