A post on the official Nintendo Facebook page states that SNES Classic pre-orders will be made available late this month (August):
We appreciate the incredible anticipation that exists for the Super Nintendo Entertainment System: Super NES Classic Edition system, and can confirm that it will be made available for pre-order by various retailers late this month.
A significant amount of additional systems will be shipped to stores for launch day, and throughout the balance of the calendar year.
As always, we’ll be tweeting out when these become available. Stay tuned!