Was going to ask the same question but i will be in Karon / Patong beach area if anyone has any idea about waves or if i can hire a board somewhere.
You could try here:
phuketwindsurfing.com/ It's a windsurfing shop, but they also rent SUPs.
Also, for general info on surfing try;
www.saltwater-dreaming.com/index.htmLast time I checked they didn't have any SUPs, but were looking into it. They may have rentals by the time you arrive.