Where can I buy good raincoats or rain jackets here in Phuket??

I'm sure at Central; Sports World has some. Also golf shops would have them. The Outlet Center on the bypass road, has a big golf shop and many other shops. I bought a heavy gage plastic rain pancho for a few hundred baht four years ago. Still works fine. I bought it a local hardware shop. I do have a rain suit, but they're hot.

There is shop north of Rassada post office in Phuket town that sells very good quality rain coats.

From Chalong going to Phuket town, turn right at the lights towards Jammin' pub (at the slaughterhouse junction where Amway is).

Next set of traffic lights turn left, go past the Rassada post office (it will be on your left), a little further down the block you will see a shop selling plates, big bins, silver pots, kitchen/ pantry stuff. Directly opposite the entrance of this shop a few steps away you will see rain coats and umbrellas hanging up. If you reach the market roundabout you've gone too far.

A set of good quality rain coat + pants cost me 350 THB, I bought it years ago and it is still in very good condition. Don't forget to haggle and good luck!

tesco have a couple of good raincoats they are around 300tbh