I usually buy PR tickets from this company in the US.
http://www.citytoursandtravel.net . They also have an office in Cebu City. It is my understanding they have a direct consolidator relationship with PR but I cannot vouch for how it plays out in the Philippines. I have only dealt with the Los Angeles office. They are the cheapest I have personally found in the US. YMMV in PI. The Cebu City office can be reached at +63-32-354-8200