Is there any travel agency in Kunming where I can buy Air Asia tickets? I've checked out several places today, but none of them sells Air Asia tickets.

You can buy the cheapest tickets on the Air Asia website.
Why would you want to go through an agency which charges a commission?

Perhaps the OP does not have a credit card to purchase the ticket. But I have to agree with such low cost carriers there is no point buying from an agent and paying a service charge. Perhaps you can approach one of your close friends to help out if they have a credit card. Could be worth a try.