Amazon sporadically restocks the Guardians of Harmony figures, and for a while Nightmare Moon and Celestia were available in incredibly limited quantities from 3rd parties that ramped the price up way beyond what they actually sell for. As of the last few days, it looks like the original three and a few new ones have been completely restocked and available directly from Amazon with Prime shipping. Have a list!
Rainbow Dash and Tank
And the three releasing on the 30th with pre-orders currently open. You might want to scoop them up to avoid the difficulties of acquiring the others:
Thanks to Micheal for the heads up!