I got it to work.
Just to recap what I did:
I added a textbox field. On each button, I set in a Rule 2 things: 1 is to set the value of my textbox field to the letter value of the button that was clicked. And 2, I query my data connection that returns clients(When creating the Data Connection, make sure the 'Automatically retrieve data...' option is not checked).
On the Listbox, for the filter, I am filtering where the clientname begins with fieldname(where fieldname is the letter value set in the rule).
Works great. Thanks!!