It wasn't hard for Tom Clancey to write about stealth aircraft in 1985. The story of stealth was broken by Jimmy Carter in 1980. He was being pounded for being weak on defense, so he revealed the existance (if not the details) of the stealth programs. Earlier, Armed Forces Journal magazine had learned of the existance of the Have Blue proof of concept prototypes, but had withheld publication at the request of the military. After Carter broke the story, AFJ published their story in their September 1980 issue. The details are available here.

In Red Storm Rising, Clancey got a lot of the plane's details wrong, he got the fundamental concept of a stealth interdiction aircraft mission right. Based on the AFJ article, that wasn't that difficult for Clancey to pull off without anyone having to tell him anything classified. There was a lot of speculation about stealth aircraft in the press. For example, an F-117A crashed in 1986 and either Time or Newsweek magazine had a great lead in sentence, "If an airplane that doesn't exist crashes in the woods, does it make a sound? Not if the Air Force has anything to say about it."

As for The Bear and the Dragon, the vehicles he described by name were Lockheed Darkstar UAVs which were currently under development. This was reported in the open press, so it wasn't hard for Clancey to include them in his book. The program was later canceled, but a "son of Darkstar" project may exist if Aviation Week and Space Technology magazine is right.