Hot water heater vents?

I have a natural gas hot water heater. The vent goes through the sheetrock to the chimney. Well I know that I have to have some space between the sheetrock and the vent pipe, which is how its set up now. But I have approx 2 inch around the vent, and cold air coming in through the opening. I have looked online and at home depot for some type of flashing, but cant seem to find anything like a sheet metal to cover the excess hole. Any suggestions?