That is not something you will be able to find at a hardware store or bike shop as an inventoried part. Like others have said, you will need to source that nut from another shifter like it. I have one of those nuts from a shifter that was broken but honestly, I have NO idea if it would be remotely the same size. The one I have came off of a thumb style early '90s shifter, its one of those shifters with the 2 thumb buttons on top of each other, not like yours.

Go to your bike coop or LBS and look for a dead shifter.