Autumn is my FAVORITE season! I love decorating for it too! One of the best places to look for bringing the natural elements of fall into our home is your own backyard! :)
Here's one idea. Have any ornamental grasses in your yard? We will cut these back a bit before the winter comes so I am more than happy to get that process started for the sake of home decor! Cut some off and pop into a vase! Mix up different "grasses" (aka: imitation looking wheat stalks right? PERFECT for the autumn season) or add any other dried items you already have.
Lookie! Doesn't it look great? Very fall-ish and it didn't cost me anything. Can't beat that type of decorating now a-days right?.........
take a peek around your yard to see what other "treasures" you can find - Branches, leaves, berries, etc. Did you plant pumpkins, indian corn, decorative gourds??....Have fun and don't limit yourself!