Important question: You said
...enter a Client ID field in a Web form field and then click on a button beside that field that will create a pop-up list of clients...
Is the list of clients that will appear *DEPENDENT* on WHAT Client ID is entered? That is, if the user enters an ID for "Walmart" does that mean that all the Walmart stores will appear? Or is the list the same for all clients? Yes, it makes a huge difference.
Oh, and what server-side technology are you using with Doofus Whacker? PHP or ASP or what?