Arundo donax is usually grown as a hardy water plant. Since ponds and water gardening are so common nowadays, I recommend calling your nearest garden centers and asking if they carry water gardening supplies and plants. If they do, they should have Arundo. Too bad you don't live in Denver! I could recommend two or three places that carry it, but if you start calling around, I don't think it will be that hard to find. Besides garden centers, try looking under water gardening in the yellow pages.

Thanks Skybird,

I hadn't thought about it being a water garden plant. The rain barrels were putting in might come in pretty handy! :)