If you have any specialty petstore, especially ones that focus on exotic animals, you might find some there.

You might check at a herp show. Though I haven't seen any sold at the one I attend, I suppose some places might.

Now alternatively, you could try getting cork like you would for a corkboard and decorate that -- gluing peat or branches to it. I'm going to be giving that a try myself. Michael's Arts & Crafts has rolls 48" long and of differing heights. My intent is to peat moss it, perhaps glue some small branches or bark to it and then have my cork bark panels over top of that. [My hope is to get a little bit of dimension and to have the board cork "fill in" the spots that the cork bark does not.] The board cork is a LOT cheaper than the cork bark, btw -- especially if you happen to have one of Michael's 50% off on item coupons.