It's part of the shifter assembly and is the stem clamp cover. You probably won't find it as a replacement item and will have to order the complete shifter assembly. Otherwise just sand or steelwool the rust off of the existing part.

That said, "stem shifters", which is what these are, are in my opinion the worst possible type. Also, stem shifters and the "suicide" brake levers mark this as an entry-level low line bike so don't put much money into new parts.