Hey Barbels,
Even though I gave you links to online places at AD, it is more fun to get it from your LFS and pick the exact shape and size you want. Only trouble is it usually costs about 3 - 4 x's more at the store.
I was in shock when I made a trip out of town a couple of weeks ago and saw prices of $50 - $75 for a smallish piece of wood. :shock:
The best place I've ever found to buy it (and choose your own) is a place like a fish or plant convention. You should see my bulging suitcases sometimes.... 8)