These kind of dolls are called Ball Jointed Dolls or the general category Dollfie.
These dolls are for sale in several web shops but to find a good deal you can search for them on Ebay too.

The key word to search for them is usually "Dollfie" or "ball+jointed+doll". Here there is the link to the full dollfie category on Ebay USA: http://dolls.listings.ebay.com/By-Brand-Company-Character_Dollfie_W0QQfclZ1QQfromZ1883QQ sacatZ84635QQsocmdZListingItemListQQsspa genameZmeQ3afQ3acat  

There are forums with collectors too and they always sell and buy dolls there.
I know a international forum (where I am registered), a spanish forum, an italian forum and a french forum.
The international forum: http://www.denofangels.com/forums/index.php

At this link  http://www.gigglegeek.dk/bjd.htm
you can find a complete database of all the ball jointed dolls with the links to every on line shop to buy them brand new!   


The used ones can be found only on Ebay or on the forums but in the forums usually you can start to trade only after a certain number of post, cause the feedback system... on the international forum you can see the marketplace section only after 45 post... so the perfect way to buy them without feedbacks is directly from the doll companies or via Ebay ^^

Here  http://www.denofangels.com/sdfaq.html   there is a nice FAQ about dolls, with many informations about them:

About customization, these dolls are highly customizable, you can buy eyes, wigs, clothes and do their face up and tattoo if you like them....

Enjoy!