It was a hard to find set for a while, but lately they are appearing in greater numbers and I believe they are shipping with the new Solomon Grundy.
His rubbery nature is a neat detail and a fun way to represent Plastic man's powers in Imaginext form.
Finally, he's so simple yet they actually sculpted the details (limited as they are) of his costume instead of just painting them on. This guy is awesome on every level!
The Martian Manhunter and Plastic Man Imaginext set is only available at Target, so keep your eyes peeled and nab it when you see it.