Drop to flat conversions work fine as long as you understand that the riding position is going to be a lot more upright with a flat bar.

I will second the Microshift recommendation. I used their 2/3 x 10 for a conversion and they work fine.

Alternatively, if you can fine a flat bar shifter for a road front derailleur, then any shimano mountain 9 speed shifter will work for the rear.

For levers: https://www.jensonusa.com/Shimano-R5...d-Brake-Levers. Cheap and work.