How about cork bark! I recently got a big piece for my turtle to use in his outside pond and it's working outstandingly. For an aquarium, I've seen it suggested that you get a piece wider than your aquarium then cut it to fit snuggly, would hold it in place (keep it from floating around). You could probably come up with other ways to anchor it if you thought about it.
A piece of bark shouldn't be considered turtle equipment. There are several places that sell it on-line.