"They seem to be really happy, but are trying to take things slow," a source tells PEOPLE

Credit: Josiah W/Splash News Online

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.