I found a great deal on it on eB*y, but that's been quite some time ago. It's available from most marine sites, and most LFS's should be able to get you some. It's fairly expensive no matter where it comes from, but you can get by with a "starter" baseball size portion. If conditions are good, it will grow rapidly. The process of nutrient export with Chaeto is by harvesting (removing) portions of it as it grows. You'll want to wait until it reaches a good size before harvesting, because a small portion won't have much affect on Nitrates/Phosphates. The ideal size would be just small enough to tumble in the refugium, but large enough to consume nutrients. It's not imperative that it tumbles, but if it does, it gets better exposure to light and water which will increase growth and efficiency.