These sponges are common in some reef tanks but I am pretty sure you can't find anyone that sells them. In fact, there is probably a lot of reefers who have them growing in their tank and they wouldn't know it. I have 2 types growing in my tank but not a lot. I can probably cut a few pieces of of one sponge and I have a rock covered in Dysidea sp.
If you want to try and seed your tank I'll be happy to give you some but you'll need to pick up.
The good news is they do grow fast with good water params.