Some of the best basing materials can be found outdoors, even in your own garden. A nice touch to any base is a tree stump, a branch, a dead tree. Unless your sculpting skills are far more superior than mine, the easiest way to achieve a realistic effect for these is to use natural materials.
Small twigs are nice to simulate tree stumps or fallen trunks, as used on my Red Karnagh Drune's base.
But to be used as a small tree, they are usually not suited. The best solution can be found underground: plant roots often have the gnarled, twisted shape one needs for a fantasy-themed base.
A good example can be seen in this WIP (Rackham's Souffre Toile from earlier this year, a project which I have abandoned for now).
Last weekend I was removing some rose bushes from my garden. I admit some of them look a bit crude since the bushes were quite big and old, but I decided to add them to my collection nonetheless.