How to do lookup with criteria

How to do lookup with criteria

I have two forms, persons, and services.  On services, I'd like to be able to choose a person.  However, I don't think combo box will work because over time this will have a lot of people in the list.
 
I can't figure out how to get a popup to work - I keep getting http 500 errors whenever I select that as an option for the lookup.
 
Thanks,
 
Keith