Some of them were actually bred there (which is a bit remarkable, given how poorly this species breeds in captivity and how absolutely atrocious this zoo's conditions appear to be).
2004 article: The Gettysburg Times from Gettysburg, Pennsylvania on March 12, 2004 · Page 32 ("Elena" the sun bear was born on March 13, 2003 at Catoctin Wildlife Preserve and Zoo. At just a few hours old and weighing a little over a pound, "Elena's" health was in question. Her mother, "Elvira," would not stop grooming the newborn. As soon as there was the slightest sign of injury to "Elena," she was taken away from her mother to insure her the best possible care.)
There's an amusing typo in that article's transcript: "For misinformation, contact Catoctin Wildlife Preserve and Zoo..."
2005 article: Catoctin Zoo brimming with babies ("...a 5-month-old Sun Bear cub named Eostair... Eostair was born at Catoctin on Feb. 12, and is among only two Sun Bears annually born in captivity, according to Schmidt. ... Catoctin Zoo is also home of two other young Sun Bears, 1-year-old Emika (Japanese for "blessed") and 2-year-old Elena ("sunshine").)
So I don't know where the original pair came from (female Elvira and a male), but it looks like they have had at least three babies - Elena, Emika, and Eostair. From other articles, Elena was the first cub born there.
A sign at the zoo states that the bears were part of a breeding program with the Miami Metrozoo (now Zoo Miami) as well as the San Diego Zoo, so perhaps one of those two zoos? I saw four of them on my visit this year.
