Go to your local river, creek, beach, lake, etc. Here you can search for driftwood and pick up niced smooth rocks that you can use to build caves and such.
I have some incredibly beautiful pieces of driftwood I have collected at some local creeks that are far nicer than any expensive piece sold in any store.
As for live plants, I really love Java Fern. It grows in almost any light and reproduces quickly. When little baby Java Ferns grow on the tips of the leaves, I pull them off and tie them to a rock using fishing line. They will then attach themselves to the rock and I can then move that rock to a new aquarium.
I have really beautiful aquariums and have never spent any more than a few dollars for a couple starter Java Ferns.
When looking for wood, look for stuff that has been wet for a while, preferably stuff that is already under water since it will already sink. If the wood floats, that is ok too and it will sink if soaked for a few weeks. Look for wood that looks very old. You can sterilize it by boiling it or running it through the dishwasher (without soap of course).
When you decorate your tank with neat rocks and wood that you have found yourself, you will enjoy it more knowing that you did it yourself instead of just buying the same old stuff at the store. . .