These are absolutely essential for batch cooking. You can use plastic freezer bags that you can buy in the supermarket. Remember that they are reusable so don’t throw them away after one use. Wash them in warm soapy water and dry well. You can also buy more durable plastic or silicone food bags online.
You will also need some glass or plastic containers for freezing. Keep the plastic containers from takeaways. These are brilliant and the perfect size for storing single portions. Glass containers with plastic lids are also great and will last a long time if you take care of them.