The Minibots and Devastator have now been listed on the Walmart site, so might be found in stores over there soon.
Devastator is US$74, while three of the four Minibots are US$8. For some reason, Walmart is scalping their own exclusives based on expected demand, by selling Bumblebee at US$9, despite the other three being "rarer" for having less units of each in the master cartons.