I would imagine there isn't a way to find that information, as these contracts are traded over-the-counter. Safe to say that it's the usual who's who of hedge funds and institutional bond holders.
from the article:
BlackRock, Loomis Sayles and Carmignac Gestion are among the asset management firms that hold Greek bonds....
also:
But with no precedent for sovereign default on publicly-held debt triggering CDS payments, many market participants have been uncertain whether their default insurance would pay out in such a scenario.
....
However, a special provision introduced soon after the 2012 restructuring says any obligation undertaken by Greecebefore Feb. 1, 2012 would not trigger CDS payments.