Advertising Directory Buttons

We have an online advertising directory that currently uses a drop down menu to display categories i.e. Restaurants, Accountants etc. The file is attached that shows this. We would like to remove the drop down menu facility and have a series of buttons A-Z instead. So if you are looking for a Restaurant for instance, you would click R and it would filter through all the Restaurants in the database. This section of the site is built around a database. If you need any further information please ask. Many Thanks