Representatives of CBS and David Letterman are still searching for a facility in Gotham to house Letterman’s new program, with the Ed Sullivan Theatre as the leading candidate.Letterman’s staff is said to be giving the Sullivan facility a second look, although there is a chance the program could wind up at the Helen Hayes Theatre among the Broadway offerings, or at CBS’ West Side Broadcast Center. A CBS spokeswoman said yesterday that no decision has been made and would not speculate on the Letterman show’s future home. However, the Sullivan theater, the longtime home of Ed Sullivan’s TV variety show, could be an appropriate venue for Letterman. In recent years, the theater played home to “Kate & Allie,” another CBS series and was used by NBC as a base for its election coverage. Because of the various configurations the Sullivan theater has gone through, exact seating capacity is unclear, but it’s believed to hold more than the 499 -seat-capacity Hayes theater.