Check out
Auto Detailing Solutions
They have very competitive prices. You can get 12pk 3.5" pads with the backing plate and 6 pk 5.5" pads for around $60. These are Buff & Shine pads. Make sure you email them asking for a 3.5" backing plate for your Porter Cable instead of a Rotary polisher. You can also check out
Car-Pro US for some other detailing stuff that Autodetailingsolutions don't have.