You can download the property owners and mailing addresses from Realist or Remine—you can also print mailing labels. Realtors get a free minimum access subscription to Remine with our MLS membership. They have great customer service. You can call them and they will tell you how to print/download the labels. Or if you are using a company to print the “just sold” post cards, they can pull the addresses for you as well. Good luck!
Your title company and sometimes a mortgage company that wants yo retain your business will do it for you and pay the postage. It's pretty expensive to do on your own.