These online shops are selling just recently released doujinshi's, and to order from them you need a shopping service like Shopping Mall Japan (Link below)
- KAC Shop
This is the site I use when I want to buy doujinshi's on Yahoo auction, and you can also place orders there from almost any online Japanese store.
- Shopping Mall Japan
The links below are shops that ship worldwide.
I have bought here countless times, and I never had any problems with them.