I'd check ebay and aquabid. other than that most lfs have a variety of ornaments to choose from. Personnally I would buy a nice piece of drift wood and attach some java moss to the top. Let it branch out over a few months and you'll have a nice shade tree. If your just looking for an easy way to create some shade in your tank nothing beats duckweed. If you want something a bit less aggressive then I'd go for a lilly.