Things From Another World, Forbidden Planet, iTunes.
Or, if you can wait until July, you can get the complete story in the omnibus book, which has now been listed on Amazon. The cover currently assigned to it follows standard IDW procedure and reuses the artwork from the first issue. Here's the blurb:
STAR TREK: The hope for the best of mankind's future! PLANET OF THE APES: A chilling look at the fall of humanity! How could these worlds possibly collide? What could possibly cause Captain Kirk and the crew of the Enterprise to side with Dr. Zaius to protect Ape City? And what does Colonel George Taylor have to say about it?
Finally, J.K. Woodward, who will be providing variant covers for the final two issues in the series, has shared another piece of art related to the crossover. Alas this amazing illustration will not be another cover, but was instead a bespoke commission for one of the series co-writers, Scott Tipton. You can see in-progress images on how Woodward created the image on his Tumblr.
To keep track of all the latest Star Trek comic releases, hit the comics button on my 2015 schedule page.