I'm going to be in Split Croatia in September of this year 2019 and would like to take a ferry to Dubrovnik. Can anyone give me some information?
Look at croatiaferries.com. It lists all the ferries and has maps.
If the ferry doesn't work out, consider the coach buses. They are nice, cheap, and frequent. You still get a great view out the window.
I don't believe there's any longer traditional ferry service between Split and Dubrovnik, but both Krilo (early morning) and Jadrolinija (mid-afternoon) run catamaran service in September. If neither of those times fits your itinerary or the sea is too rough for the catamarans to sail, comfortable Pullman coach buses depart regularly throughout the day.