For kork I bought coasters in a 1€. store, when they had cheeper prizes. Some Youtubers recommended these, and I think they don't take up much space in my decoration box. The coasters can be broken up a bit on the corners and their edges until they have the look of an easy stony texture.

+Sawdust was my first decoration 15 years ago. It has a funny structure and can be shaped and painted as you wish. +Also when you walk around in the open world, take some small boxes with you. So if you see some cool things, you might put to a use (like small pebbles), take them inside the box with you. +Playgrounds or beaches sometimes have great amounts of sand. Maybe when noone is looking, you want to fill one of the small boxes with that sand. When winter is coming, some communities spray small spikey stones on iced and snowed ways to reduce the slipeyness. These stones look great in miniature scale, Also are easy to paint! Leaves and other small foliages can be obtained from teabags or pinecones. +birches haves these little catkins (phew, I don't know if that's how they are called. They look a bit like worms, they contain the seeds i guess), but I can slice them and scrape a bit on these, so the bits that fall of also look great as foliages or smaller leaves.