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.
Step 2: Cut Along the Seam of the Tube.
You now have a really long flat piece of rubber.
Step 3: Cut the Rubber into Yarn.
Cut a thin strip up the long length of the rubber.
When you get to the top, turn a right angle with your scissors, continuing to follow the outline of the rubber strip.
Keep going with your scissors, following a spiral pattern, moving inwards, until you have one huge length of rubber yarn.
When you cut round the corners, keep the cut as rounded as you can.
The corners will have little excess flappy bits. You can tidy them up later if you want, depending on how bothered you are about neatness in your end knitting.
The thinner you cut the strips, the longer your yarn will be.
Different thicknesses make for interesting knitting!