Electroneum is not yet listed at an exchange.
The Electroneum blockchain only goes live on 1 November 2017, and after that it will take a few days/weeks for exchanges to test their systems before accepting Electroneum.
According to the Electroneum FAQ and also electroneumexchange.net, they are currently in talks to have it listed on Cryptopia, Yobit, and Liqui.io.
For the time being, you’ll have to find a seller yourself and perhaps meet up in person.