I get my manga and comics through DCBS. Since DC Comics is CMX’s parent company, CMX manga is listed on the site like any other back list trade paperback. So it is easy to pick up older issues of CMX manga there for around 40-35% off. The site navigation to find these deals is a little confusing, if you click on “Trade Paperbacks”, then “DC”, then use the sort by publisher bar at the top you can see the issues of CMX manga they have in stock. I added three volumes of Moon Child to my usual monthly order.
Updated to add: DCBS has a sister site called In Stock Trades where they also sell their backlist. Shipping is free if you order over $50 worth of books. So if you are picking up bunch of older stuff at once, it might be better to order over there and save on the shipping. Here’s their CMX listing. I do my usual monthly order through DCBS and just add 2-3 backlist titles in here and there when I’m light on pre-orders, and my shipping is usually around $7.00. If you needed to pick up most of Moon Child all at once ordering through In Stock Trades might be a better option. In Stock Trades has selected volumes of Viz and Tokyopop manga listed too, although I think that they tend to have more complete runs of series for CMX manga.