I was wondering where I could get P.O Box in Chiang Mai, and what documents they'll be needing from me?

I've had a box at the Chiang Mai University Post Office for more than three years and I think I only needed my passport to get it. And I was on a tourist visa at that time too. Don't know if requirements have changed since then. I had to pay extra for the key but it was a one time purchase.

Some Thai post offices have boxes available to rent depending on which one you go to. So if you find the post office closest to you does not have boxes to rent try one of the other post offices. When I had a Thai post office box rent was about 500 a year, the prices will vary depending on the size box you rent. Once you find a post office with available boxes, your passport and baht should be all you need to rent the box.

If you are an American and are retired military or 100% military disabled, you may also be eligible for a free box at the U.S. Consulate in Chiang Mai. The mail room there is only open on Mondays and Wednesdays from 10:00 AM to Noon, come to the entrance that faces the Ping River during that time, They close at Noon so do not wait until the last minute. Bring you military retirement ID, Passport and DD-214 as well as one copy each, If you are not retired military but 100% military disabled also bring your letter of disability.