This isn't an official statement, rather just my thoughts/speculation based on what I know:
As long as the Serpentor sets and HISS Tanks are sitting on shelves collecting dust, there will be no restock. Basically, when TRU or Walmart runs out, they place another order for the SKU #. If there is no more wave 1 stock, they will get wave 2. They don't order specific waves, rather just product based on the SKU.