1) Potatoes - not just any though!
2) egg box
4) 'special knowledge' (you'll get that in a minute...)
Step 1: Buying the potatoes
You can pick them up fairly cheaply from a good shop - mine here cost Â£2 from the local nursery
More importantly is that you can often pick up varieties of potato not usually available or even endangered or rare plants - though often these are sold as actual plants rather than the rare and valuable tubers.