Not sure about SC itself, but if you come north just a bit, into the San Jose area there are a LOT LOT LOT of landscape and masonry stores that sell all sorts of rock from all over the world. Whether you actually will find something labeled "Seiryu" is questionable, but with so many stores, and so much to choose from you will probably find some stone you like even if the store calls it something else. You may actually find real Seiryu from a geology point of view. That is, the same minerals in the same formation. Just harvested somewhere else. (And probably named for that location)
One of the better places is Peninsula Building Supply. It is not the only one, by far! It is worth the trip.
You might look around closer to you, first. Look for:
Landscape, Masonry, Brick, Soils, Bulk materials, Sand, Concrete (maybe) and related terms. Call them and ask not for Seiryu Stone (No body will know what that is) but just to see if they carry decorative rock by the pound. Go get a sample. Often there are broken off chunks that you can have. Bring some baggies and a sharpie to label your samples. Then go home and test them in water to see if they will alter the GH, KH, pH or TDS of your water. If the tests are OK for your fish, go get the rocks!