Call the music stores in the area and find out which one has the largest selection of PA speakers in the area. Visit the store and ask to see their selection of PA speakers.
Tell the attendant what you are trying to power or how many people are in your band. This will let the attendant know what kind of power or volume that you need. It is also important to tell them what kind of PA you have and how much power it can take. This will narrow your search even more.
Choose the PA speakers that will best serve your band. Take into account the kind of venues where you might perform. This will help you decide how many speakers you will need. A regular coffee shop-type venue would require at least two column speakers and one floor monitor. A larger venue would require larger speakers and more floor monitors.