Carbon.

[snakefing]

What you want are fast-growing things that can fix carbon from the CO2 in the air. That'd be plants mostly. Animals and fungi get their carbon mostly from their food, so you could skip the middleman and harvest the carbon from their food directly.

Maybe giant algae vats? Big shallow vats or pools, stirred to keep the algae well-supplied with CO2. This could provide significant amounts of biomass, but it would take a lot of space and sunshine.

[Túrin]

I GOT IT.

URINE.

Me and my friend one time had to pee in a bucket (long story) and forgot about it in his attic. When we rediscovered it months later, nothing was left but this black stuff that he said was "carbon". There could be... urine farms!

And if you have the people eat the plastic, you'd have a closed biological cycle!

[Stargate525]

hell, if you want it for a setting, make one up.

A crystalline race of beings whose structure is almost completely carbon, making them almost as hard as diamonds. Then, since they are crystals and can't move, make enormous farms of them and harvest to your hearts content.

[the_taken]

CUT-> NO! I didn't post anything. It was an accident! Nothing is here. Nothing is here.

[Poseptune]

There is a substance that was found on a planet that can be used to make metal with similar properties to that of steel, but is more durable. It is a turquoise crystal, that is readily availible with some dangerous mining.

[Master Hagortough]

you can get carbon this way