Dead bike inner tubes are in abundance, ask your nearest bike shop or community bike workshop to give you some.
Not only can you knit (or crochet, sewing or whatever-else) for free, but you are reusing a waste material.
It is inspired by my favourite Instructable by Marcellahella- Knit With Inner Tube Thread
I have a few adjustments though, I feel my method makes the yarn a bit stronger.
You will need:
1 dead bike inner tube.
Scissors. (Not your best ones, this will blunt them rapidly.)
Piece of scrap cardboard.
Wooden knitting needles. (Preferably wooden but not essential.)
Step 1: Cut the Air Valve Out of the Tube.
Cut the air valve out of the tube by snipping straight across the tube either side.
You don't need the air valve now.
You are now left with a long tube of rubber.