Update: Contrary to an initial statement we received from a GameStop representative, GameStop made the NES Classic Edition available online, both individually and through bundles, at 12am ET. Individual consoles sold out on the site a couple hours later then came back around 10:10am. As of 11:40am, they are sold out again.
Best Buy made the system available online around 1:20am ET. The device was sold out by 10:30am, but came back in stock around 1pm. Since then, the device has oscillated between being available with a ship date of July 20 and being available for store pickup only.