By scouring neighborhood construction sites, you can find lots of great garden elements. Large boulders, bits of broken rock, and other structural items can usually be gained for free from construction sites. Once you have acquired these materials, you can create a garden walkway or an Zen-type garden space.
If you're strapped for cash, look for bargain items on barter websites. Often, homeowners will give away various materials (such as brick, stone, and other items) that are no longer needed. In addition, these sites are great for purchasing or bartering for a couple of healthy plants.