"Exapostilarion" is how it is spelled in Bishop Kallistos' texts. It is also spelled this way in the Boston texts, and in the Menaion and Octoechos by St. John of Kronstadt Press.
Exapostilaria beats Exaposteilaria by 535:340
Wikipedia also has it's article spelled this way:
Frjohnwhiteford 15:45, April 15, 2007 (PDT)