4th Dimension Entertainment has announced their Rarity plush! She will be available in stores and online this coming fall. From their Twitter:
Rarity will be in stores / online this fall! The gems aren't included, but if you do have any lying around darling...They recently posted photos of their Applejack and Big McIntosh plush as well, although they did not give an estimated date on those two.
Twilight Sparkle and Pinkie Pie are currently available for sale on their site.