There are some silicone anemones - see these YouTube videos:
Aquarium Decoration Tutorial: How To Make a Silicone Bubble Tip Anemone
Fake Anemone Hosting Clownfish
Though these look pretty pathetic and I would prefer a flower pot in my DT tank over these.
The problem is that they made them solid and didn't sculpt them very realistically.
If you would make them hollow with a thin wall (think about it like a condom but a bit thicker) and sculpt it more realistically you cold come pretty close in look and feel for the clownfish.
After all an anemone is in essence not much more than a organic bag filled with water - that's what makes them so fragile.