Hello, I would just tear newspaper into the smallest pieces possible, and use some old bark from trees.
Best of luck
Mike

Sawdust or wood shavings, if you know a woodworker or cabinet shop. Sawmills make mountains of the stuff. Make sure it's not treated - the most common place for that to turn up is a hardware store or lumber yard that does cutting for customers.

Wood chips. Electric utilities have trimming crews that sometimes will dump a load in your driveway if you call them and get on a list.

If you have a city sponsored yard waste dropoff or mulch site, that stuff is usually heavy on the browns.

I get shredded wood mulch for free from my little city. They collect the tree/brush trimmings continuously through the year, shred it into mulch, make piles of varrying age from fresh to ~ 2 years, and residents can have 2.5 cu yards per quarter for free.

Also check with local tree services.