Remarkable amounts of carbon are available in waste streams, even if you exclude from the count plastics and other petrochemicals. Paper, cardboard, wood, natural fibers, carbon in sewage and waste food, and especially farm waste (parts of plants not otherwise consumed). Some of the latter is needed for soil conditioning, but most of that is from decay of roots, not stuff left at the surface.

All this can be extended by addition of hydrogen. Naively, if you process a carbohydrate into hydrocarbons, about half the carbon is lost as CO2. Adding hydrogen allows the oxygen to be carried off as water rather than CO2 (or, the CO2 to be converted to hydrocarbons and water in a second step.) Hydrogen currently comes from natural gas but that will have to change anyway, with the hydrogen being produced by (for example) electrolysis of water.