In Chiang Mai, you can visit a handful of boxing stadiums where Muay Thai fights are organized almost every evening.
A bit north: Chiang Mai Boxing Stadium
In the Night Bazaar: Kalare Boxing Stadium
In the red light district (right between the girl bars): Loi Kroh Boxing Stadium

And if you’re interested, there are many Muay Thai schools as well: just google ‘chiang mai muay thai school’ and you’ll find several dozen. Pick a few and see which atmosphere you like best.

An overview can be found here.