Auto-Populate A Pop-Up Menu in Numbers 3.5
I had the same sort of problem and with some help from people within the Apple Support community I came up with this solution. Hopefully it will work for you.

This is my list of players in sheet 1. In your case, the Players name would be the item codes.



Here is sheet where I want to information copied to.


The P# corresponds to the number next to the Players name in Sheet 1. If I enter a 1 and a 2 the names are filled in to the column to the left of the box.
 


This shows the formula required to transfer the players name from sheet 1 to sheet 2. 


To take it a step further, when I fill in the Players winnings in Sheet 1 it also fills it in all the "Earnings" cells for that player in Sheet 2 using the formula shown below.




My list of players goes to 300 but the last 20 or so are empty. That way I can still add more names or in your case, item codes, to it.

This may be more complicated than you require but hopefully it helps you.