I've only ever purchased popular configurations but from what I've tried to put together before online, it seems like that only popular base models or maxed out versions are kept in store. Anything that has a something like a base RAM but maxed out hard drive, etc is usually custom order. I could be way off but that's my experience when trying to shop for an MBP. If you configure your build online, it should show you at the end which store it's available at if they do indeed carry it. If not, it'll give you an estimate of how long it'll take to receive or go pickup once it does arrive.