as a frequent visitor to Myanmar I normally book all my internal flights online with one of the many internal operators Air KBZ.Golden Myanmar,Asian Wings,Air Bagan etc etc.
For International flights ,originating in Mandalay or Yangon, I can recommend two TA's who I have found very reliable
http://onestopmyanmar.com/
and
http://www.sevendiamondtravels.com/
SS