I just read this in a locked thread:
You're only chance of getting any Mac & adding BTO (build-to-order) options such as an upgraded HD or RAM is either direct through Apple.com or by calling up Smalldog.com & placing it through them. Other than that, I believe you can only get the "Stock" models through other e-tailers. It's just always been that way with every Mac really. You're only way to customize is through Apple.com & pay the stupid sales tax.
While I'm shocked that somebody used "you're" incorrectly, there are a few things that all Apple customers need to know when shopping for a machine.
1) Most Apple Resellers do not advertise CTO (a/k/a Configured-To-Order) systems for several reasons (which I shouldn't have to explain to you all). That does NOT mean that they are unable of selling you a CTO box. You just may have to wait a while to get it. Almost ANY authorized reseller can do this. Additionally, you should inquire about any CTO machines sitting inventory. You might be surprised at the number of people who have canceled their order after the computer has already been delivered to the reseller.
2) Call the vendor if at all possible (I recommend this on any low-volume product). Many web sites exclude certain low-inventory items from showing up. This was common at MacWarehouse, and it's common elsewhere. If you must know why, think about it. Sometimes a company gets ONE shipment, and that's it. Sometimes it's only one system, and they will not be able to re-order. Web orders for these kind of things can be dodgy on items like that.