If you want a cheap flight from Chiang Mai to Tokyo, you've got one airlines to choose from.
You can use the filters on your search to see more flights and find cheaper rates – Tokyo has more than one airport. So if you don't need to fly into New Tokyo International Airport, it's worth checking your other options. The 4470 km flight usually takes about 15 hrs 50 mins.
During this peak season, accommodation prices often increase too, so it's best to book early. Or you could keep your fingers crossed and hold out for a last-minute deal.