Where is a good place to buy a decent rain coat that doesn't break the 2nd time I wear it? I've checked Big C, Lotus, Robinson, some sporting shops, the entire Suan Kaew mall, and a couple small markets. So far, all I've seen is the super cheap plastic and the kind that button up in the front which are worthless on a motorcycle.

I think that you will find that with a raincoat you will become too hot, too quickly since there is no airflow. I have two good quality raincoats I bought in Europe but every time I have tried to wear one here, I became so hot within the first ten minutes that I could no longer wear one - even the very thin one. I haven't noticed anyone here wearing a "proper" raincoat. I guess that is why.

I second the question, I am a biker too, have been scouting the town to find something good. Even front closed plastics are hard to find. Good for heavy rain and longer trips. There MUST be something. Ok plastic makes you wet from the inside because of transpiration,

With all the heavy bikes around, I doubt that one must go to Bkk or ship in from abroad.

Shops and brand recommendations would really really help.

Alternatively, is there a good mail order source ?

Try any of the motorbike helmet shops around the outside road of the moat. Mine cost me around 200 Baht, from memory, for a silver-lined one with hood and a zipper at the front.

If worried about overheating, a big umbrella will do the job. Or just find a shelter and wait it out. Most down-pours in Chiang Mai don't last very long. Enjoy the cooler and cleaner air while you wait