Match zip code to city

I have a very large database that i am frequently adding to and it involves entering addresses of buildings. My problem is i get tired of going through the database to find the zip code that matches that city should it not be given to me. I would like to enter the city and it search down the city column and inserts a matching zip code in the correct column. At this point i have almost every city and its zip code in the database, so i don't need to worry about creating a whole different database of many city and zips to reference.

My Layout
Column M: Lists the City
Column O: Where i want the zip code to be inserted

Thanks in advance to anybody who is able to help me out.