Echoing the above...
Take a walk in the woods, you'll find most of what you need and if you walk along a creek you can get ideas of how to set it up as well. Grab some plants/moss while you're out there.
Having said that... I've also gotten stuff from landscape yards. There have been times where I wanted slate or cut rock that nature wasn't providing. Also, for my aquariums I tend to want what they call broken or scrap. So for several years I had a relationship with a local supply yard where I donated a few toys to their Christmas toy drive and I got all the aquarium rocks I wanted for "free".
Win/Win/Win