Originally Posted by
Retro Grouch
Might be easier than you think.
Look for a handlebar that has a 25.4 mm clamp. That'll match your existing stem and at least some of those had a smaller grip size so your thumb shifters will work.
Note that flat bars have a 22.2mm diameter on most of the bar (with 25.4mm or 31.8mm on the clamp area).
Drop road bars have 23.8mm clamp section on the non-clamp part of the bar.
So your mtb shifters on the current flat bar have a 22.2mm clamp diameter.
You can get a road bar with 25.4mm clamp diameter (and it will fit on the current stem) but taht won't take care of your shifter problem.