YES there are raw materials for making plastics other than found on earth(as far as we explored the solar system). Since major of raw material of the plastics are from the petrochemicals (they are chain of carbons and other organic compounds).
It is possible to grow genetically engineered plants in space to extract raw material for making plastics
We've engineered a new metabolic pathway in plants for producing a kind of fatty acid that could be used as a source of precursors to chemical building blocks for making plastics such as polyethylene
Rich organic compounds are found in the meteorites and also in the space dust
Ethane and methane are found in the lakes of titan but they must processed to convert them into a polymer