The retaining wall blocks from Timbercrete are often in stock and available on special order.

The wall block is 155mm by 400mm by 200mm and 16 blocks per square metre weighing approximately 13kg.  
The special order Curved wall blocks weigh 11kg. Each block locks up approximately 1.82kg  of carbon. 200mm wide cobblestone mortared retaining wall blocks are also available.

The wall block landscape series tapers back due to the 32mm wide key, and can be stacked without the need for mortar. For now these will be special order, but Timbercrete plans to have stock in the future.
The curved block is also available in this series, for curved retaining wall construction.

Dry stack retaining wall blocks available from some suppliers.