You generally need to go through another PR firm or someone else already in the industry. The industry is pretty insular so they prefer to take calls from others within the industry.
It is possible to do some searches on google to find out which firms represent which celebs. You could then try to simply contact that firm's main office. An upfront approach can work because it illustrates that you have a legitimate reason. However, that could still prove to be difficult.
I would recommend contacting any leading PR firm to establish a relationship and then use that firm to contact your target.