What are the most efficient ways of obtaining polymer in single player on the island?
Solution 1:
Farm organic polymer - it acts as normal one, except it spoils after 30mins.
Find Kairuku around north-west part of Island and hit it with Wooden Club for best results.
If you need "large amounts" for some reason - in singleplayer you could increase Harvest - quantity of items gained in settings. But organic polymer should be enough.