Amazon.co.jp: they have a huge selection and good prices, but unfortunately international shipping is air express only (fast, but costs about 40$ and up, depending on how much you order).
Japanese HMV: good prices, good shipping, but they are always out of stock of everything. I ended up cancelling an order with them because they were like "you can wait till the stuff comes into stock and we'll keep renewing your order" but it was obvious that was just a way of saying they had nothing.
***NEW PLACE I found***
has all of Vampires, Ludwig B (just ordered it!), Black Jack, and Pluto, as well as some anime like Cleopatra and Belladonna. Their prices are all converted from yen. The shipping is not quite as bad as amazon.co.jp, but still pretty expensive if you get a registered package.
If anyone knows of any other reliable sources for Tezuka manga in Japanese with good prices and/or shipping (ESPECIALLY for more rare stuff!), or if you have reviews of other sources for Tezuka manga in Japanese, please reply