The National Ballet of Canada is looking for a new artistic director. But that individual won't be able to start work until Dec. 31, 1989, because the company is giving new two-year contracts to its two current artistic directors, Valerie Wilder and Lynn Wallis. American-born choreographer Glen Tetley also agreed to continue as artistic associate for the next two years, the company said. National Ballet spokesman Gregory Patterson said Wallis and Wilder had no specific plans for the end of their tenure. "I think they just felt four years was enough," he said.