If you are interested in getting into Asian Ball-Jointed Dolls or just want to learn more about them, most of these sites really helped me when I got started in this hobby. I would really recommend visiting Den of Angels if you have any interest at all in BJD's, and make sure to do plenty of research before buying anything. There are a lot of terms to learn and things to decide before buying one.
Forums and Informational Sites
Den of Angels
Resinality
BJD WTF
Modern Asian Ball-Jointed Dolls
Company Sites
There are many, many different websites where you can purchase the dolls and/or accessories, I'm just listing a few to help get started. For a complete list, I would recommend visiting the websites above.
Everything of Doll - Dollmore
Dream of Doll
Fairyland
SOOM
Iplehouse