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Apple hasn't said anything; but if previous generations are any indication, the display models should arrive around the time they start arriving to pre-order customers. Generally speaking, products become available in-store for display on "launch day", though this model doesn't yet have a firm "launch day". For example, even though many stores didn't get even a single iPhone 7 plus jet black due to the demand and manufacturing issues; they still had display models.

In the past I've seen display Mac's appear a few days before the first customers began to get them. But I would still expect mid-November before we see them on display in stores. Probably December/January timeframe before you can just walk in and buy one; without having pre-ordered it for in-store pickup. Folks are already seeing mid to late December shipping times.
 
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