Preparing for a bond issue this spring to finance construction of its new headquarters, NPR got a vote of confidence from two of the big-three bond rating agencies, the network said in a news release yesterday. Standard & Poor’s gave NPR an AA- rating and Moody’s gave it a comparable Aa3. Both are high-grade ratings, the fourth of 20 or more grades. Last March, NPR bought the site seven blocks east of its present home and is planning a new seven-story, 330,000-square-foot structure incorporating about two-thirds of an old historic-landmarked warehouse. NPR plans to break ground next fall and occupy the building by mid-2013.