Water World Asia (formerly Dive Supply) and All-4-Diving are the biggest and best stocked stores in Phuket. Both are in Patong and they are both quite comprehensive in their selection of all types of gear and general dive supplies and toys. WWA main lines are Scuba Pro, Sea and Sea, and Suunto, while A4D carries allot of Aqua Pro gear. There are also several smaller shops in Patong, including Diver's Emporium and Warm Water Divers.
While the selection of brands and depth of stock is very good in Patong/Phuket (it is likely you will find whatever you need), the prices are quite high when compared with USA or Singapore (the 2 places I know directly). Compared to Singapore, prices are anywhere from 25-40% more expensive.
Also note that the selection of gear for UW photography (housings, lenses, strobes, etc) is very limited and EXPENSIVE. WWA sells some Sea and Sea cameras and strobes and you can find housings for some Sony and Canon cameras around the island.