You can buy them on Radius' website for $7.95, and replacable heads are $6.95. However, there are other green toothbrushes that are cheaper, such as the Preserve which costs $13 for 4. The other benefit of the Preserve is that you can send it back to the makers and they will recycle it for you. ::Radius
The Source toothbrush helps to cut waste by using a reusable handle made of wood fiber, blended with a plastic derived from Nebraska maize. Into this you can put disposable heads, which cuts down on 4/5ths of waste when you replace it, compared to throwing away a traditional toothbrush.