Asus newbie

Sorry if this sounds ridiculous, but where can I get a customized Asus ultraboook? Every other time I've bought a laptop (Lenovo and Apple) I've just ordered from the company, and been presented with a myriad of options. Now, I'm trying to buy the UX21A, but I can only find it at Best Buy, Target, etc, and none of them have the configurations I need. :( Help me please?