There's LOTS of sites, but for preorders the 3 most popular ones for new item preorders are
http://www.amiami.com/
http://www.hlj.com/
http://www.1999.co.jp/eng/
They will usually give a 10-20% discount off mspr on preorders. All 3 are based in Japan, and will be much cheaper and get their products faster than shops based in the US. I wouldn't recommend buying from US shops unless that's your only option because they are sold out at the Japanese shops.
Big exception: Kotobukiya's Bishoujo figures, these will almost always be cheaper in the US, and will be in stock at amazon with free super saving/prime shipping.
Preorder discounts here are only 5%, but their sale section has good deals on recently released figures that aren't selling that great.
http://plamoya.com/
Once something is sold out, your only option to get it is preowned, and for that I mostly use
http://www.amiami.com/top/page/c/preowned.html
http://ekizo.mandarake.co.jp/shop/en/
Shining Wizard @ Sawachika is the sculptor of that Tharja figure. That guy is probably my single favorite sculptor. Did Catherine, Zero Suit Samus, and all the 7th dragon 2020 figures. The guy does great work.
Another pic.