Hey Matt,
You don't generally see them for sale except sometimes at frag swaps.
I have a lot of sponge growing under my rocks and some yellow growing on the tops of rocks. The stuff I have is not easy to frag off, it has just grown over the years.
I know I saw some sponge for sale as frags at the last swap in columbus. I think someone from the club bought some, not sure how it did or if they can frag it.
--Ray