Was walking the dog and popped into a local shop for some plant supplies and stumbled upon a whole grocery isle of fun stuff for the tank at good prices, including bags of empty shells of all sizes. It's called Jamali Floral & Garden 149 West 28th St. (between 6th & 7th). I just bought a huge piece of barnacle, wrapped up in plastic for $10. I washed it off and have it "curing" in saltwater in my otherwise empty QT. Will let it sit there for the next week. My Gobies are gonna love it.