There are a few good sites , the ones just of the top of my head are
www.mediamix.com
www.detailanddesigninscale.com
www.hlj.com (hobbylink japan)
www.raggededgedesigns.com
www.modelcraftsandhobbies.co.nz
www.hiroboy.com
www.modelcraftersgp.com
then there are a whole raft of other places , some places offer different parts that others don't or you can get something cheaper on a another site .
plus I hardly ever buy a kit where I live as it is cheaper to get it sent from overseas at the fastest postage.
Hope this helps
Shayne