Hey, so you can book them online over the official websites of the ferry companies or in Dubrovnik port there is Jadrolinija ticket shop for ferry tickets with Jadrolinija company. But you can easily buy them online, also spots are limited so in summer season always try to book tickets in advance if you know when you will be traveling. 

Check these guides for islands to visit near Dubrovnik, they also have guides on where and how much the tickets cost:

Lopud: How to spend one perfect day on this island (Great island to visit from Dubrovnik)

Dubrovnik to Korcula Day Trip: How to Reach it and What to see

Lokrum Island Guide: All You Need to Know Before Visiting

If you need other guides on Dubrovnik check this ultimate guide which has everything you need: Ultimate Dubrovnik guide (Best overall, compiled)