I am VERY new to this, but I only know of one store downtown. Watercolors Aquarium Gallery on Jefferson. I don't recall seeing live rock there, but I also wasn't looking.
Blue Fish Aquarium in Grandville seems to always have a good selection of live rock when we are there. I haven't purchased live rock so I can't speak to their quality. We have purchased all of our saltwater livestock from them. So far we haven't had any issues. Their saltwater staff is very helpful and knowledgable.
