Tom
as has been said there's plenty of natural stuff to be had, obviously this depends on what you are planning.
A good source for stumps with roots is seaweed, find the type that has a thick stem and anchors to rocks with rootlike branches. The best bits are the ones that have dried out above the tideline (no green issues with these). These have a nicely wrinkled surface for a rotted trunk look.
Garden bark chippings are good th create stone walls and slabs etc.
On the sloghtly dafter side M&S has a cheap pack of 4 columns for wedding cakes that can be broken down and provide convincing debris.
Loads of seedheads from gardens, stones of all sizes from roadside gutters, a small pinch between thumb and forefinger will give you a natural range of sizes.
For bushes you can pull up a small ash sapling, shake and wash soil off, turn upside down and voila instant bush, if you want leaves on it then you have to decide how detailed you want to be. A good halfway is to dab the branches (roots) with PVA and sprinkle oregano over it, makes for a decent display standard but would not hold up to competition for that it's leaf by leaf I'm afraid.
Basically just use your imagination, groundwork is my favourite part of the hobby.
Cheers
Derek