You can buy any Apple product in the Apple Stores. As for customizing it, your best bet is to order it online as the Apple Store employees will have to order for you anyway since they won't have anything other than the stock models.
Shop for your Apple, Mac, iPhone and other computer products on Amazon.
We are a participant in the Amazon Services LLC Associates Program, an affiliate program designed to provide a means for us to earn fees by linking to Amazon and affiliated sites.