I was wondering if anyone knew where to purchase manga online? I don't mind if the site and manga is in Japanese. So far the sites I've come across only sell the most popular titles and do not sell any of the manga I'm looking for.

Well, there's Girlfriends on Amazon if that counts

You can also get Kisses, Sighs and Cherry Blossom Pink in English on Amazon now. Word is that School Police is coming soon.

honto.jp probably has was you're looking for and has the best international shipping rates.
amazon.co.jp also has a good selection.