I've never encountered an actual Apple Store not having specced up machines on hand (maybe not EVERY possible configuration, but a decent selection). If you go to a non-Apple retailer, afaik, they are limited to the base spec machines (RAM wise), it's not even a matter of them being out of stock. If you want anything other than 8GB (or whatever the base amount is for that model) of RAM you need to go through Apple directly. This was the case, at least in Canada a few years ago, correct me if I'm wrong/things have changed.