"It is I" is definitely the grammatically correct choice. However, you are correct that it is not spoken very often (especially in U.S. English). I would restate your example to say, "I was salesman of the month." Then your writing is grammatically correct without having to worry about sounding awkward. I remember even as a child in school, I understood correct grammar as something other than what I heard people speaking around me. It's very useful for formal writing, but if you speak with Americans you will hear many variations (most of which are perfectly acceptable in most situations, since Americans in general tend to be rather informal.)