Converting early '90's Trek 1400 from traditional road bars to a flat/straight mtn bar. Converted V-brakes over to use Deore brake levers. Works well. Need to get the shifters up to the bars, replacing the originals on the down tube. Which shift control/assembly do need ? (original Shimano 105 components).
For the rear, any Shimano shifter (for the right number of speeds) will work - the cable for the rear is the same between mountain and road. On the other hand, the front derailleurs have different cable pulls, and you need the flat bar road shifter (one of these). You can get the Sora set for 8 speed assuming your bike has an 8 speed cluster in the back (Shimano ST-R221 for the front , Shimano SL-R440-8 for the back). They are pricey though compared to their mountain brethren, and you only need the front one from that pair - the rear can be an 8 speed Alivio or something, but it wont look as nice (but function fine).
Some alternatives for the front: depending on if you swap in a closer spaced crankset, you may be able to get a mountain FD to shift it and get a mountain set [depending on how the FD is mounted]. Or more simply, buy a friction shifter for the front and an index shifter for the back. A friction shifter is like 5 bucks for a thumb mounted one.
Also, note that the riding position does change when you switch from drops to flats, so I'm assuming you've worked out those details already.