Somewhere like Cost Plus, World Market or anywhere that sells wicker baskets. You can find something the right size and shape.
But you will probably need to line it with a napkin or something because the basket weave is usually too open and makes unmolding the dough on to the peel a bit tricky. But they are cheap.
I bought my bannetone (did I spell that right?) at Sur la Table for about $25 I think.