Here are a few sites that are useful when you want to purchase the manga or other things.

Ebay.com
Not many SB! items are sold there but sometimes you can find the Japanese shoujo manga magazine "Hana no Yume" (where the newest Skip Beat! chapters are being published) and manga cd's or other stuff.

Ebay.de
At the German ebay you'll find a lot of the Daisuki volumes (the Daisuki is a German shoujo manga magazine that publishes the newest Skip Beat! chapters) and of course the German edition of the Skip Beat! mangas.

Japan-Manga.jp
This is great place to buy the original Japanese Skip Beat! mangas or other stories by Yoshiki Nakamura

Sasuga Japanese Bookstore
Great online store where you can find lots of mangas, artbooks and japanese goodies.

Yahoo Auctions Japan
The best place to find Skip Beat! stuff but most of the sellers won't ship internationally therefore I would recommend you to use Rinkya although it's a bit expensive.

Tuttocomics.ch
You can buy there a Skip Beat! clear file (if you are living in Europe).