This is a tough question. If the sale as has been recent, which generally is from 2004 onward, it will show up in most online real estate websites. You will be able to get the names of the parties involved. Prior to that it is very difficult. Co-op sales were not recorded as real estate transactions, as it was a transfer of shares in a corporation. You could try good old-fashioned sleuthing - i.e. check the name on the bell. If you really need to get in touch you can write a letter to "Owner" at that address with that apt. number.