Jennifer Morrison and Sebastian Stan
Josiah W/Splash News Online
07/17/2012 AT 05:00 PM EDT
There may be a happily ever after for two stars of Once Upon a Time
Following in the footsteps of Ginnifer Goodwin and Josh Dallas – whose relationship became public
in April – fellow costars Jennifer Morrison, 33, and Sebastian Stan, 28, are an item, too.
"They met on set and are now exclusive," a source tells PEOPLE. "They started dating three months ago. They seem to be really happy together, but are trying to take things slow."
This isn't the first time either half of the pair has gotten cozy with a costar.
Stan formerly dated Leighton Meester
when they were both on Gossip Girl
, and Morrison was engaged
to her former House M.D.
castmate Jesse Spencer.
Stan, who has only been in three episodes of Once Upon of Time
, also stars in the USA miniseries Political Animals
, alongside Sigourney Weaver.