Natural sponges DO come from the sea. Yes.
But these long scrubber type sponges are actually the skeletons of fruiting vegetables you can grow yourself (and eat mid-summer!)! Here's an old blog post from my family/garden blog that tells all about it. Now is the time to put in your seed order if you want to plant luffa gourds to eat and then save to dry and use as sponges.
If you're going to grow luffas for use as sponges, look for "dishcloth" or "sponge" or "smooth" luffas. They are wider in circumference than the "angled" luffas used in Chinese cuisine. Many seed suppliers have luffa seeds, but my favorite company, Baker Creek Heirloom Seeds, definitely carries a wide variety.
I'm Wendy Kiang-Spray, gardener, home cook, and author of The Chinese Kitchen Garden. Learn more about the book here. Enjoy the blog and be sure to like The Chinese Kitchen Garden Facebook page for notifications when there are new posts.