Aargh! You made me research.

I reckon their clothes are made of man-made fibre, like nylon, which you can make at home. I can't imagine where they keep field of cotton or herds of sheep. Though you can make fibre from lots of things - like possum fur. Depends on which animals survive and how they can be kept from destruction by the machines.

Really, the resistance is way too well-equipped for the apocalypse.